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Does pregnancy alter life-course lipid trajectories? Evidence from the HUNT Study in Norway

  • Amanda R. Markovitz
    Correspondence
    To whom correspondence should be addressed.
    Affiliations
    Department of Epidemiology Harvard T.H. Chan School of Public Health, Boston, MA

    Connors Center for Women's Health and Gender BiologyBrigham and Women's Hospital and Harvard Medical School, Boston, MA
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  • Eirin B. Haug
    Affiliations
    Department of Public Health and Nursing, Norwegian University of Science and Technology, Trondheim, Norway
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  • Julie Horn
    Affiliations
    Department of Public Health and Nursing, Norwegian University of Science and Technology, Trondheim, Norway

    Department of Obstetrics and Gynecology, Levanger Hospital, Nord-Trøndelag Hospital Trust, Levanger, Norway
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  • Abigail Fraser
    Affiliations
    Population Health Sciences, Bristol Medical School and MRC Integrative Epidemiology Unit at the University of Bristol, Bristol, UK
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  • Corrie Macdonald-Wallis
    Affiliations
    Population Health Sciences, Bristol Medical School and MRC Integrative Epidemiology Unit at the University of Bristol, Bristol, UK
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  • Kate Tilling
    Affiliations
    Population Health Sciences, Bristol Medical School and MRC Integrative Epidemiology Unit at the University of Bristol, Bristol, UK
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  • Eric B. Rimm
    Affiliations
    Department of Epidemiology Harvard T.H. Chan School of Public Health, Boston, MA

    Department of Nutrition, Harvard T.H. Chan School of Public Health, Boston, MA

    Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA
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  • Stacey A. Missmer
    Affiliations
    Department of Epidemiology Harvard T.H. Chan School of Public Health, Boston, MA

    Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA

    Division of Adolescent and Young Adult Medicine, Department of Pediatrics, Boston Children's Hospital and Harvard Medical School, Boston, MA

    Department of Obstetrics, Gynecology, and Reproductive Biology, College of Human Medicine, Michigan State University, Grand Rapids, MI
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  • Paige L. Williams
    Affiliations
    Department of Epidemiology Harvard T.H. Chan School of Public Health, Boston, MA

    Department of Nutrition, Harvard T.H. Chan School of Public Health, Boston, MA
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  • Pål R. Romundstad
    Affiliations
    Department of Public Health and Nursing, Norwegian University of Science and Technology, Trondheim, Norway
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  • Bjørn O. Åsvold
    Affiliations
    K.G. Jebsen Center for Genetic Epidemiology, Norwegian University of Science and Technology, Trondheim, Norway

    Department of Endocrinology, St. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway
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  • Janet W. Rich-Edwards
    Affiliations
    Department of Epidemiology Harvard T.H. Chan School of Public Health, Boston, MA

    Connors Center for Women's Health and Gender BiologyBrigham and Women's Hospital and Harvard Medical School, Boston, MA

    Department of Public Health and Nursing, Norwegian University of Science and Technology, Trondheim, Norway

    Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA
    Search for articles by this author
Open AccessPublished:October 12, 2018DOI:https://doi.org/10.1194/jlr.P085720
      We examined the association between pregnancy and life-course lipid trajectories. Linked data from the Nord-Trøndelag Health Study and the Medical Birth Registry of Norway yielded 19,987 parous and 1,625 nulliparous women. Using mixed-effects spline models, we estimated differences in nonfasting lipid levels from before to after first birth in parous women and between parous and nulliparous women. HDL cholesterol (HDL-C) dropped by −4.2 mg/dl (95% CI: −5.0, −3.3) from before to after first birth in adjusted models, a 7% change, and the total cholesterol (TC) to HDL-C ratio increased by 0.18 (95% CI: 0.11, 0.25), with no change in non-HDL-C or triglycerides. Changes in HDL-C and the TC/HDL-C ratio associated with pregnancy persisted for decades, leading to altered life-course lipid trajectories. For example, parous women had a lower HDL-C than nulliparous women at the age of 50 years (−1.4 mg/dl; 95% CI: −2.3, −0.4). Adverse changes in lipids were greatest after first birth, with small changes after subsequent births, and were larger in women who did not breastfeed. Findings suggest that pregnancy is associated with long-lasting adverse changes in HDL-C, potentially setting parous women on a more atherogenic trajectory than prior to pregnancy.
      Lipids predict future CVD (
      • Di Angelantonio E.
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      • Kaptoge S.
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      Major lipids, apolipoproteins, and risk of vascular disease.
      ). Although recent evidence suggests that altering HDL cholesterol (HDL-C) levels does not itself cause changes in CVD risk (
      • Rosenson R.S.
      The high-density lipoprotein puzzle: why classic epidemiology, genetic epidemiology, and clinical trials conflict?.
      ), lower HDL-C may serve as a marker of a more atherogenic lipid profile. Pregnancy induces profound metabolic, endocrine, and cardiovascular changes that may have long-lasting or permanent effects in mothers. In the first 10 years after giving birth, changes in lipid levels have been observed (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Knopp R.H.
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      • Van Allen M.I.
      Lipoprotein metabolism in pregnancy, fat transport to the fetus, and the effects of diabetes.
      ,
      • Potter J.M.
      • Nestel P.J.
      The hyperlipidemia of pregnancy in normal and complicated pregnancies.
      ). Although total cholesterol (TC) appears to return to prepregnancy levels within a year (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • van Stiphout W.A.
      • Hofman A.
      • Bruijn A.M.D.
      Serum lipids in young women before, during, and after pregnancy.
      ), there is consistent evidence that HDL-C decreases postpartum and remains lower than prepregnancy levels for multiple years (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • van Stiphout W.A.
      • Hofman A.
      • Bruijn A.M.D.
      Serum lipids in young women before, during, and after pregnancy.
      ,
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ,
      • Skilton M.R.
      • Bonnet F.
      • Begg L.M.
      • Juonala M.
      • Kähönen M.
      • Lehtimäki T.
      • Viikari J.S.A.
      • Raitakari O.T.
      Childbearing, child-rearing, cardiovascular risk factors, and progression of carotid intima-media thickness.
      ) and less consistent evidence that triglycerides remain elevated postpartum (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ). However, no study has examined lipid trajectories beyond a decade after pregnancy (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • van Stiphout W.A.
      • Hofman A.
      • Bruijn A.M.D.
      Serum lipids in young women before, during, and after pregnancy.
      ,
      • Skilton M.R.
      • Bonnet F.
      • Begg L.M.
      • Juonala M.
      • Kähönen M.
      • Lehtimäki T.
      • Viikari J.S.A.
      • Raitakari O.T.
      Childbearing, child-rearing, cardiovascular risk factors, and progression of carotid intima-media thickness.
      ). If adverse changes in lipids continue into midlife and beyond, it could provide insight into the early origins of subclinical CVD risk in women. In addition, few previous studies were able to characterize how lipid levels changed with time since pregnancy. Furthermore, few studies have considered breastfeeding as part of the peripartum year. Lactation is a modifiable factor that might minimize adverse changes in lipids postpartum and is associated with higher HDL-C levels (
      • Knopp R.H.
      • Walden C.E.
      • Wahl P.W.
      • Bergelin R.
      • Chapman M.
      • Irvine S.
      • Albers J.J.
      Effect of postpartum lactation on lipoprotein lipids and apoproteins.
      ,
      • Erkkola R.
      • Viikari J.
      • Irjala K.
      • Solakivi-Jaakkola T.
      One-year follow-up of lipoprotein metabolism after pregnancy.
      ,
      • Kallio M.J.
      • Siimes M.A.
      • Perheentupa J.
      • Salmenperä L.
      • Miettinen T.A.
      Serum cholesterol and lipoprotein concentrations in mothers during and after prolonged exclusive lactation.
      ,
      • Kjos S.L.
      • Henry O.
      • Lee R.M.
      • Buchanan T.A.
      • Mishell D.R.
      The effect of lactation on glucose and lipid metabolism in women with recent gestational diabetes.
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      • Natland S.T.
      • Nilsen T.I.L.
      • Midthjell K.
      • Andersen L.F.
      • Forsmo S.
      Lactation and cardiovascular risk factors in mothers in a population-based study: the HUNT-study.
      ) and a more rapid return of triglycerides to prepregnancy levels (
      • Darmady J.M.
      • Postle A.D.
      Lipid metabolism in pregnancy.
      ). A previous study found that women who breastfed longer had smaller postpartum decreases in HDL-C (
      • Gunderson E.P.
      • Lewis C.E.
      • Wei G.S.
      • Whitmer R.A.
      • Quesenberry C.P.
      • Sidney S.
      Lactation and changes in maternal metabolic risk factors.
      ).
      The population-based Nord-Trøndelag Health Study (HUNT), linked with the Medical Birth Registry of Norway, includes data on pregnancy, breastfeeding, and measured lipid values in women from before and up to 41 years after first birth, enabling an examination of changes in lipid levels pre- to postpregnancy. These data enable us to examine, for the first time, the impact of pregnancy on the ratio of TC to HDL-C, which performs as well as or better than other lipid measures in CVD risk prediction (
      • Manickam P.
      • Rathod A.
      • Panaich S.
      • Hari P.
      • Veeranna V.
      • Badheka A.
      • Jacob S.
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      Comparative prognostic utility of conventional and novel lipid parameters for cardiovascular disease risk prediction: do novel lipid parameters offer an advantage?.
      ,
      • Mora S.
      • Otvos J.D.
      • Rifai N.
      • Rosenson R.S.
      • Buring J.E.
      • Ridker P.M.
      Lipoprotein particle profiles by nuclear magnetic resonance compared with standard lipids and apolipoproteins in predicting incident cardiovascular disease in women.
      ,
      • McQueen M.J.
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      • Volkova E.
      Lipids, lipoproteins, and apolipoproteins as risk markers of myocardial infarction in 52 countries (the INTERHEART study): a case-control study.
      ,
      • Kastelein J.J.P.
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      • Cater N.B.
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      • Deedwania P.
      • Olsson A.G.
      • Boekholdt S.M.
      • Demicco D.A.
      Lipids, apolipoproteins, and their ratios in relation to cardiovascular events with statin treatment.
      ,
      • Ingelsson E.
      • Schaefer E.J.
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      • McNamara J.R.
      • Sullivan L.
      • Keyes M.J.
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      • Wilson P.W.F.
      • D'Agostino R.B.
      Clinical utility of different lipid measures for prediction of coronary heart disease in men and women.
      ,
      • Ridker P.M.
      • Rifai N.
      • Cook N.R.
      • Bradwin G.
      • Buring J.E.
      Non-HDL cholesterol, apolipoproteins A-I and B100, standard lipid measures, lipid ratios, and CRP as risk factors for cardiovascular disease in women.
      ,
      • Natarajan S.
      • Glick H.
      • Criqui M.
      • Horowitz D.
      • Lipsitz S.R.
      • Kinosian B.
      Cholesterol measures to identify and treat individuals at risk for coronary heart disease.
      ). Using these data, we investigated the association of first birth with short- and long-term changes in lipid levels. We also examined the impact of later births on lipid levels. Finally, we investigated the extent to which these changes differed by breastfeeding length.

      MATERIALS AND METHODS

      Study population

      HUNT is a population-based open cohort study of adult Nord-Trøndelag county residents designed for a wide range of health-related research. County-wide surveys are conducted roughly every decade, with three completed at the time of this analysis. This analysis was restricted to the second and third surveys in which lipids were sampled: HUNT2 (1995–1997) and HUNT3 (2006–2008). During the surveys, participants received an extensive health assessment that included blood sampling, clinical measurements, and questionnaires (
      • Krokstad S.
      • Langhammer A.
      • Hveem K.
      • Holmen T.L.
      • Midthjell K.
      • Stene T.R.
      • Bratberg G.
      • Heggland J.
      • Holmen J.
      Cohort profile: the HUNT Study, Norway.
      ). All current county residents aged 20 years or older identified from the national population register were invited to participate in each survey, with participation rates among women of 76% in HUNT2 (
      • Holmen J.
      • Midthjell K.
      • Krüger Ø.
      • Langhammer A.
      • Holmen T.L.
      • Bratberg G.H.
      • Vatten L.
      • Lund-Larsen P.G.
      The Nord-Trøndelag Health Study 1995–97 (HUNT 2): Objectives, contents, methods and participation.
      ) and 59% in HUNT3 (
      • Krokstad S.
      • Langhammer A.
      • Hveem K.
      • Holmen T.L.
      • Midthjell K.
      • Stene T.R.
      • Bratberg G.
      • Heggland J.
      • Holmen J.
      Cohort profile: the HUNT Study, Norway.
      ). Residents of Nord-Trøndelag county are predominantly white and generally representative of Norway as a whole (
      • Holmen J.
      • Midthjell K.
      • Krüger Ø.
      • Langhammer A.
      • Holmen T.L.
      • Bratberg G.H.
      • Vatten L.
      • Lund-Larsen P.G.
      The Nord-Trøndelag Health Study 1995–97 (HUNT 2): Objectives, contents, methods and participation.
      ).
      We linked HUNT data to the Medical Birth Registry of Norway, which includes all births in Norway from 1967 (
      • Irgens L.M.
      The Medical Birth Registry of Norway. Epidemi­ological research and surveillance throughout 30 years.
      ) through the end of our data collection in 2012. Because older women were unlikely to have their pregnancy history captured in this registry, we restricted analyses to women aged 20–60 years during lipid measurement. Figure 1 outlines the process of identifying two overlapping study populations. The first included parous and nulliparous women with similar age distributions to compare lipids trajectories between the two groups. The second included parous women to compare lipids before and after first birth. For the first population, we excluded women born before 1940 or after 1974 to prevent misclassification of women as nulliparous who had a birth before the birth registry started in 1967 or after the end of data collection in 2012. We applied this exclusion to parous women to achieve a comparable age distribution. For the second population of only parous women, we did not restrict based on birth year. All participants in HUNT signed an informed consent form allowing the use of their data and samples for research. This project was approved by the Central Norway Regional Committee for Medical and Health Research Ethics and was considered exempt from institutional review board review by the Harvard T. H. Chan School of Public Health. This study abides by the Declaration of Helsinki principles.

      Lipid and covariate assessment

      Participants' ages ranged from 20 to 60 years during measurements. Among parous women, 2,488 women had at least 1 measurement before their first birth (with a total of 2,521 prepregnancy observations), including 747 women with measurements both before and after first birth (supplemental Fig. S1). Nonfasting lipids were measured from serum samples. For technical details about TC, HDL-C, and triglyceride measurements, see supplemental Table S1. We calculated non-HDL-C as TC minus HDL-C and the TC/HDL-C ratio. LDL cholesterol (LDL-C) was not analyzed because the Friedewald formula (
      • Friedewald W.T.
      • Levy R.I.
      • Fredrickson D.S.
      Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge.
      ), typically used to calculate LDL-C, performs poorly in nonfasting samples (
      • Scharnagl H.
      • Nauck M.
      • Wieland H.
      • März W.
      The Friedewald formula underestimates LDL cholesterol at low concentrations.
      ,
      • Jun K.R.
      • Park H-I.
      • Chun S.
      • Park H.
      • Min W-K.
      Effects of total cholesterol and triglyceride on the percentage difference between the low-density lipoprotein cholesterol concentration measured directly and calculated using the Friedewald formula.
      ,
      • Lindsey C.C.
      • Graham M.R.
      • Johnston T.P.
      • Kiroff C.G.
      • Freshley A.
      A clinical comparison of calculated versus direct measurement of low-density lipoprotein cholesterol level.
      ). At the time of lipid measurements, staff recorded the time since last meal in hour categories.
      Covariates were collected during HUNT surveys and were selected based on the causal diagram shown in supplemental Fig. S2. We used data from all HUNT questionnaires, including HUNT1 (1984–1986), to identify the following time-invariant covariates: 1) family history of CVD (any reported myocardial infarction or angina pectoris in siblings or parents); 2) smoking status at the age of 20 years, defined as ever versus never smoked daily prior to or at the age of 20 years, to approximate prepregnancy smoking behavior for parous women; and 3) highest obtained education level. HUNT3 did not collect education level and was instead derived from work titles for 13% of women based on recommendations from Statistics Norway (
      Statistics Norway.
      ). We also included the following time-varying, or updated, covariates: 1) BMI, 2) smoking status, 3) alcohol use, 4) vigorous leisure-time physical activity, and 5) oral contraceptive use. Time-varying covariates were measured at the time of lipid assessment either from HUNT2 and HUNT3 clinical examinations (in the case of BMI) or HUNT2 and HUNT3 questionnaires (for all other covariates). We obtained additional information about first births, including maternal age and preterm status (<37 weeks gestation), from the birth registry and breastfeeding length after first birth (self-reported as 0, <3, 3–6, and >6 months) from HUNT questionnaires. We additionally obtained self-reported information about menopause transition and hormone replacement therapy (never, previous, or current user) from HUNT questionnaires.

      Analysis

      We used linear mixed-effects models to estimate lipid trajectories as a function of age, accounting for the timing of a woman's first birth. Age was modeled using restricted cubic splines with four knots located at ages 23, 37, 46, and 57 years based on prespecified quantiles of the age distribution, as recommended by Harrell (

      Harrell, F. E., 2010. Springer, New York.

      ). Two variables were used to estimate the effect of pregnancy. The first indicated whether measurement preceded or followed the first birth, providing an estimate of short-term change in lipids after first birth. The second indicated continuous time since first birth, providing an estimate of longer-term change in lipids postpartum. All models controlled for the participant's age at measurement, HUNT survey (HUNT2 vs. HUNT3), time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      First, we compared life-course lipid trajectories for parous and nulliparous women based on completed reproductive history. In this analysis (see supplemental Methods section 1), nulliparous women represented background age and secular trends independent of parity. These analyses included an indicator of final parity status (i.e., none vs. one or more births; covariate Pi in supplemental Methods equation 1) and an interaction between parity and the spline terms, allowing the age-related splines to differ throughout the life course based on final parity status. We chose final rather than updated parity status because nulliparous and parous women are likely to be different even before the latter give birth, given observed associations between infertility and lipid levels (
      • Kim J.J.
      • Choi Y.M.
      Dyslipidemia in women with polycystic ovary syndrome.
      ,
      • Mahalingaiah S.
      • Sun F.
      • Cheng J.J.
      • Chow E.T.
      • Lunetta K.L.
      • Murabito J.M.
      Cardiovascular risk factors among women with self-reported infertility.
      ). For these and other analyses controlling only for baseline covariates, we used a complete case analysis, excluding participants with missing data on education (0.6%) or smoking (2.5%). To present the trajectories graphically, predicted lipid trajectories were derived for hypothetical nulliparous women and parous women with a first birth occurring at the age of 23 years (the median age at first birth in the study population), setting all other covariates to their mean levels.
      Second, we used the same mixed-effects spline models among parous women (study population 2) to obtain estimates of the short-term effect of the first birth on lipid levels and to describe differences in this effect estimate by length of breastfeeding (supplemental Methods section 2). The short-term effect of the first birth was estimated based on the discontinuity between predicted trajectories before first birth and predicted trajectories after first birth (captured by the coefficient Iij in supplemental Methods equation 2). Fully adjusted results from these models included updated BMI, alcohol use, physical activity, and oral contraceptive use as well as maternal age and preterm status of first birth. Both baseline and updated covariates were multiply imputed using fully conditional specification (
      • van Buuren S.
      Multiple imputation of discrete and continuous data by fully conditional specification.
      ) with 25 iterations. Models investigating whether breastfeeding length moderated the change in lipid levels from before to after first birth included distinct indicator terms for the postpartum versus prepartum effect based on breastfeeding length. Approximately 99% of lipid measurements informing after first birth trends in our analysis occurred after breastfeeding ended; thus, the before versus after first birth effect we estimated would include changes associated with breastfeeding. We used F-tests to determine whether the postpartum versus prepartum effect differed by breastfeeding length (
      • Li K-H.
      • Meng X-L.
      • Raghunathan T.E.
      • Rubin D.B.
      Significance levels from repeated p-values with multiply-imputed data.
      ). Finally, we investigated the change in lipids after second and third births among women with multiple births.

      Sensitivity analyses

      We performed sensitivity analyses among women with measurements at both HUNT2 and HUNT3 (47% of women) to verify that our main results, which included some women with only one measurement, were consistent with within-woman changes in lipids observed among women with more than one lipid measurement. First, we replicated the lipid trajectory models among women with repeated measures to verify that our results could be interpreted as within-woman life-course trajectories. Second, we compared the within-woman change in lipid levels from HUNT2 to HUNT3 for women who had one or more births during the ∼11 year interval to the within-woman lipid changes for women who remained nulliparous during the interval using a difference-in-differences approach (
      • Imbens G.W.
      • Wooldridge J.M.
      Recent developments in the econometrics of program evaluation.
      ). We additionally performed sensitivity analyses controlling for menopause transition and hormone replacement therapy to see whether this differentially affected life-course lipid trajectories for parous compared with nulliparous women. All analyses were performed using Stata IC 13 (StataCorp, College Station, TX) and MLwiN (
      • Rasbash J.
      • Charlton C.
      • Browne W.J.
      • Healy M.
      • Cameron B.
      MLwiN Ver. 2.34.
      ) version 2.34.

      RESULTS

      Among 21,312 study participants born between 1940 and 1974 (study population 1), 8% were nulliparous throughout the follow-up period, at which point the youngest were 38 years old. Nulliparous and parous women had similar age distributions, but nulliparous women were less likely to smoke or consume alcohol and more likely to participate in vigorous physical activity (Table 1). Nulliparous women were slightly more likely to be obese and had greater levels of nonparticipation and missing data.
      TABLE 1Description of covariates at the individual and observational level based on final parity status among HUNT2 and HUNT3 study participants born between 1940 and 1974 (n = 21,312 women)
      Final Parity Status
      NulliparousParous
      Women (n)1,62519,687
      Birth year [median (IQR)]1959 (1951–1967)1958 (1951–1966)
      Smoking status at the age of 20 years [n (%)]
      Never smoked daily919 (57)9,043 (46)
      Ever smoked daily653 (40)10,166 (52)
      Not reported53 (3)478 (2)
      Education [n (%)]
      Lower secondary309 (19)3,164 (16)
      Upper secondary672 (41)9,185 (47)
      Tertiary604 (37)7,244 (37)
      Missing40 (2)94 (0)
      Family history of CVD [n (%)]504 (31)6,611 (34)
      HUNT exam participation [n (%)]
      Only HUNT2751 (46)6,827 (35)
      Only HUNT3288 (18)3,333 (17)
      Both HUNT2 and 3586 (36)9,527 (48)
      Age in years at first birth [median (IQR)]N/A23 (20–26)
      Births [n (%)]
      1N/A2,171 (11)
      2N/A8,804 (45)
      ≥3N/A8,712 (44)
      Breastfeeding length of first birth [n (%)]
      Did not breastfeedN/A1,120 (6)
      <3 monthsN/A3,086 (16)
      3-6 monthsN/A5,503 (28)
      >6 monthsN/A7,462 (38)
      MissingN/A2,516 (13)
      Preterm first birth [n (%)]N/A1,187 (6)
      Observations
      Observations reflect HUNT survey periods. Individual women who participated in both HUNT2 and HUNT3 contributed two observations.
      (n)
      3,32931,743
      BMI at HUNT exam [n (%)]
      <25 kg/m21,674 (50)15,728 (50)
      25–29.9 kg/m2963 (29)10,826 (34)
      ≥30 kg/m2665 (20)5,136 (16)
      Not available27 (1)53 (0)
      Alcohol consumption [n (%)]
      <1 glasses per 2 weeks1,162 (35)9,277 (29)
      1–4 glasses per 2 weeks1,167 (35)14,797 (47)
      ≥5 glasses per 2 weeks932 (28)7,221 (23)
      Missing68 (2)448 (1)
      Vigorous physical activity [n (%)]
      <1 h per week1,669 (50)18,327 (58)
      ≥1 h per week1,183 (36)9,921 (31)
      Missing477 (14)3,495 (11)
      Oral contraceptive use [n (%)]
      Nonuser2,024 (61)22,716 (72)
      Current user564 (17)2,961 (9)
      Missing741 (22)6,066 (19)
      a Observations reflect HUNT survey periods. Individual women who participated in both HUNT2 and HUNT3 contributed two observations.
      Predicted life-course lipid trajectories, based on multivariable models, suggested higher HDL-C levels among parous women before first birth compared with nulliparous women (Fig. 2A), with a difference of 2.0 mg/dl (95% CI: −0.1, 4.2) at the age of 20 years (Table 2). However, the HDL-C levels of parous women dropped at first pregnancy and thereafter were lower or equal to those of nulliparous women (Table 2). HDL-C in parous women after their first birth never returned to the same levels that would be predicted given the observed prepregnancy slope (Fig. 2A). Using the full population of parous women, the estimated short-term drop in HDL-C after first birth was −4.2 mg/dl (95% CI: −5.0, −3.3) (model 1, Table 3), representing a 7% decrease in HDL-C compared with average prepregnancy levels. This decrease dropped to −3.0 mg/dl (95% CI: −4.2, −1.8) after adjusting for updated covariates, including BMI (model 2, Table 3).
      Figure thumbnail gr2
      Fig. 2Predicted lipid trajectories and 95% CIs based on final parity status for (A) HDL-C, (B) non-HDL-C, (C) triglycerides, and (D) TC-HDL-C ratio. Estimates for parous women are predicted for women with a hypothetical first birth at the age of 23 years. Gaps represent pregnancy and 3 months postpartum. Predictions are at the mean level for the following covariates: HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of cardiovascular disease.
      TABLE 2Predicted lipid levels by age based on final parity status for HUNT2 and HUNT3 participants born between 1940 and 1974 (n = 21,312 women)
      Lipids
      Adjusted for participant's age at measurement, HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      NulliparousParous with First Birth at the Age of 23 YearsDifference (Parous − Nulliparous)
      Estimate95% CIEstimate95% CIEstimate95% CIP
      HDL-C (mg/dl)
      20 years53.751.6, 55.855.854.8, 56.72.0−0.1, 4.20.06
      30 years55.054.0, 56.053.152.6, 53.7−1.8−2.8, -0.8<0.001
      40 years57.656.7, 58.555.855.4, 56.2−1.8−2.7, -1.0<0.001
      50 years60.559.5, 61.559.158.7, 59.6−1.4−2.3, −0.40.006
      60 years61.359.5, 63.161.360.6, 62.00.0−1.8, 1.80.99
      Non-HDL-C (mg/dl)
      20 years136.0129.9, 142.1130.6127.8, 133.4−5.4−11.6, 0.70.09
      30 years141.6138.7, 144.5135.4133.9, 136.9−6.2−9.0, −3.3<0.001
      40 years153.1150.5, 155.7148.2146.9, 149.5−4.9−7.4, −2.5<0.001
      50 years172.5169.6, 175.4167.6166.3, 169.0−4.9−7.7, −2.1<0.001
      60 years194.0188.5, 199.5188.0185.9, 190.0−6.1−11.6, -0.60.03
      Triglycerides (mg/dl)
      20 years101.390.4, 112.299.694.5, 104.6−1.7−12.7, 9.20.76
      30 years100.295.0, 105.394.391.6, 97.1−5.9−10.9, −0.90.02
      40 years103.398.6, 108.1100.297.8, 102.7−3.1−7.6, 1.40.18
      50 years115.3109.6, 120.9116.9114.3, 119.51.6−3.8, 7.00.56
      60 years133.8122.5, 145.2137.9133.8, 141.94.0−7.4, 15.50.49
      TC/HDL-C ratio
      20 years3.73.6, 3.93.53.4, 3.6−0.23−0.40, −0.060.008
      30 years3.83.7, 3.83.73.7, 3.8−0.05−0.13, 0.040.28
      40 years3.93.8, 3.93.93.8, 3.9−0.01−0.09, 0.070.78
      50 years4.14.0, 4.24.14.0, 4.1−0.03−0.12, 0.050.45
      60 years4.44.3, 4.64.34.3, 4.4−0.11−0.26, 0.050.17
      a Adjusted for participant's age at measurement, HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      TABLE 3Differences in lipid levels from prepregnancy to after first birth by breastfeeding length for parous HUNT2 and HUNT3 study participants (n = 22,047 women)
      Model 1
      Adjusted for participant's age at measurement, HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      Model 2
      Additionally adjusted for updated BMI, updated alcohol use, updated physical activity, updated oral contraceptive use, maternal age at first birth, and preterm first birth.
      Estimate95% CIPEstimate95% CIP
      HDL-C (mg/dl)
      All parous women−4.2−5.0, −3.3<0.001−3.0−4.2, −1.8<0.001
      By breastfeeding length
      Did not breastfeed−6.2−7.3, −5.1<0.001−3.9−5.3, −2.5<0.001
      Breastfed <3 months−5.2−6.1, −4.3<0.001−3.5−4.7, −2.2<0.001
      Breastfed 3-6 months−4.0−4.9, −3.1<0.001−2.7−3.9, −1.5<0.001
      Breastfed >6 months−3.8−4.6, −2.9<0.001−2.9−4.1, −1.7<0.001
      Non-HDL-C (mg/dl)
      All parous women−0.4−2.8, 2.10.771.1−2.3, 4.50.53
      By breastfeeding length
      Did not breastfeed5.62.3, 8.8<0.0013.7−0.3, 7.70.07
      Breastfed <3 months2.2−0.5, 5.00.112.1−1.5, 5.70.25
      Breastfed 3-6 months0.5−2.0, 3.10.671.5−2.0, 5.00.39
      Breastfed >6 months−1.9−4.4, 0.50.130.3−3.1, 3.80.85
      Triglycerides (mg/dl)
      All parous women−3.6−7.8, 0.70.101.8−3.8, 7.50.52
      By breastfeeding length
      Did not breastfeed9.74.1, 15.4<0.0017.50.8, 14.20.03
      Breastfed <3 months1.9−2.9, 6.70.433.8−2.2, 9.80.22
      Breastfed 3-6 months−2.6−7.1, 1.90.251.8−4.0, 7.70.54
      Breastfed >6 months−6.6−10.9, −2.30.0030.7−5.1, 6.40.82
      TC/HDL-C ratio
      All parous women0.180.11, 0.25<0.0010.160.06, 0.25<0.001
      By breastfeeding length
      Did not breastfeed0.430.33, 0.52<0.0010.270.16, 0.39<0.001
      Breastfed <3 months0.290.21, 0.37<0.0010.210.11, 0.31<0.001
      Breastfed 3-6 months0.190.12, 0.26<0.0010.150.06, 0.250.002
      Breastfed >6 months0.120.06, 0.19<0.0010.130.04, 0.230.006
      Models include all parous women without restricting to participants born between 1940 and 1974, as in Fig. 2 and Table 2 comparisons with nulliparous women.
      a Adjusted for participant's age at measurement, HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      b Additionally adjusted for updated BMI, updated alcohol use, updated physical activity, updated oral contraceptive use, maternal age at first birth, and preterm first birth.
      Non-HDL-C was lower in parous compared with nulliparous women (Fig. 2B, Table 2), with no meaningful changes at first birth (Table 3). Triglycerides were similar in the two groups (Fig. 2C, Table 2) and did not meaningfully change after first birth. As expected, given lower non-HDL-C levels and higher HDL-C levels, the TC/HDL-C ratio was lower in parous women before their first birth compared with nulliparous women [−0.23 units (95% CI: −0.40, −0.06)] (Table 2). After first birth, this ratio increased by an estimated 0.18 units (95% CI: 0.11, 0.25) among parous women (model 1, Table 3). In the decades after first birth, the TC/HDL-C ratio for parous women was similar to the ratio for nulliparous women of the same age, with no indication of a return to the lower, more favorable ratio experienced by parous women before their first birth (Fig. 2D, Table 2). Changes in lipid levels before to after first birth did not differ based on maternal age at first birth or gestation length of first birth (results not shown).
      The majority of parous women breastfed for at least 3 months (Table 1), and longer breastfeeding times were associated with less adverse changes in all lipid levels (Table 3). The changes in lipid analyte levels before to after first birth were moderated by breastfeeding length in all models included in Table 3 (P < 0.01; results not shown). However, women who breastfed >6 months after first birth still experienced a decrease in HDL-C and an increase in the TC/HDL-C ratio after first birth, even after adjusting for updated covariates (model 2, Table 3). After restricting analyses to women with two or more births, we found similar changes in HDL-C and the TC/HDL-C ratio after first birth, but changes after subsequent births were smaller in magnitude (Table 4).
      TABLE 4Differences in lipid levels from prepregnancy to after birth by birth order for parous HUNT2 and HUNT3 study participants with at least two births
      Adjusted for participant's age at measurement, HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      Women with 2+ Births (n = 18,956)Women with 3+ Births (n = 8,976)
      Estimate95% CIPEstimate95% CIP
      HDL-C (mg/dl)
      First birth−3.6−4.8, −2.5<0.001−2.7−4.8, −0.60.01
      Second birth−1.6−2.4, −0.7<0.001−2.0−3.4, −0.50.008
      Third birth−2.2−3.2, −1.3<0.001
      Non-HDL-C (mg/dl)
      First birth2.7−0.6, 6.10.113.6−2.4, 9.70.24
      Second birth−1.4−3.9, 1.10.270.8−3.6, 5.10.73
      Third birth0.2−2.6, 3.10.86
      Triglycerides (mg/dl)
      First birth−0.3−6.1, 5.50.925.9−4.2, 16.00.25
      Second birth−2.2−6.6, 2.30.340.1−7.4, 7.60.98
      Third birth0.1−5.0, 5.20.98
      TC/HDL-C ratio
      First birth0.220.12, 0.31<0.0010.180.02, 0.340.03
      Second birth0.03−0.05, 0.100.490.09−0.03, 0.210.12
      Third birth0.08−0.00, 0.160.05
      a Adjusted for participant's age at measurement, HUNT survey, time since last meal, education, smoking initiation by the age of 20 years, and family history of CVD.
      Sensitivity analyses among the subset of women with repeated lipid measurements yielded similar predicted lipid trajectories, although with wider CIs due to the lower statistical power (supplemental Fig. S3). Women with a birth between HUNT2 and HUNT3 experienced a decrease in HDL-C and an increase in TC/HDL-C compared with women who did not give birth during the ∼11 year period, with a magnitude similar to associations seen in the main analysis (supplemental Table S2). Lipid trajectories were also similar after adjusting for menopause and hormone replacement therapy use (supplemental Fig. S4).

      DISCUSSION

      We found that women who bore children had more positive lipid profiles before first birth compared with women who remained nulliparous throughout life. However, this advantage changed at first birth, when new mothers had a decrease in HDL-C and an increase in the TC/HDL-C ratio compared with nulliparous women. Our findings suggest that adverse changes in HDL-C and TC/HDL-C associated with pregnancy persist across a woman's life course. In addition, the specificity of the timing of the change in these lipid profiles suggests that pregnancy itself causes the drop in HDL. Adverse changes after pregnancy were present after adjusting for a variety of behavioral changes associated with a first birth, including higher BMI, lower alcohol use, less physical activity, and less oral contraceptive use. Although longer breastfeeding appeared to moderate adverse changes in HDL-C and the TC/HDL-C ratio after pregnancy, women who breastfed for >6 months still experienced worse lipid profiles postpartum compared with prepregnancy levels.
      The magnitude of the drop in HDL-C associated with first pregnancy of −4.2 mg/dl is consistent with previous studies that examined short-term changes in lipids from prepregnancy to postpartum (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ). In the longitudinal Coronary Artery Risk Development in Young Adults (CARDIA) Study, HDL-C differed by −3 to −4 mg/dl over an interval of 2–8 years for women who had a first birth during the interval compared with a nulliparous reference group (
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ). Using a study design similar to CARDIA, adolescents in the National Heart Lung and Blood Institute's Growth and Health Study who had a first birth during a 10 year interval had a −4 mg/dl decrease in HDL-C compared with a nulliparous reference group. The similarity of estimates in an adolescent population is consistent with our finding that the changes in lipids associated with pregnancy did not differ based on maternal age at first birth. These studies similarly found that the changes after pregnancy were specific to HDL-C (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ,
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • van Stiphout W.A.
      • Hofman A.
      • Bruijn A.M.D.
      Serum lipids in young women before, during, and after pregnancy.
      ,
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ), although one study found elevated triglycerides postpartum (
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ). Our study obtained similar results despite different strengths and weaknesses compared with previous studies. For instance, CARDIA had repeated measurements for all participants but examined changes only up to 10 years, while our study expanded measurement up to 40 years postpartum but included a mixture of cross-sectional and longitudinal data.
      Our study is the first to report the association between parity and the TC/HDL-C ratio, which has been found to have a stronger independent predictive power for CVD over its component lipid measures (
      • Manickam P.
      • Rathod A.
      • Panaich S.
      • Hari P.
      • Veeranna V.
      • Badheka A.
      • Jacob S.
      • Afonso L.
      Comparative prognostic utility of conventional and novel lipid parameters for cardiovascular disease risk prediction: do novel lipid parameters offer an advantage?.
      ,
      • Ingelsson E.
      • Schaefer E.J.
      • Contois J.H.
      • McNamara J.R.
      • Sullivan L.
      • Keyes M.J.
      • Pencina M.J.
      • Schoonmaker C.
      • Wilson P.W.F.
      • D'Agostino R.B.
      Clinical utility of different lipid measures for prediction of coronary heart disease in men and women.
      ,
      • Ridker P.M.
      • Rifai N.
      • Cook N.R.
      • Bradwin G.
      • Buring J.E.
      Non-HDL cholesterol, apolipoproteins A-I and B100, standard lipid measures, lipid ratios, and CRP as risk factors for cardiovascular disease in women.
      ,
      • Hsia S.H.
      • Pan D.
      • Berookim P.
      • Lee M.L.
      A population-based, cross-sectional comparison of lipid-related indexes for symptoms of atherosclerotic disease.
      ), providing a more holistic atherogenic index. Based on this measure, while parous women lose their initial advantage of high HDL-C and low non-HDL-C after their first birth, their long-term TC/HDL-C trajectory is no worse than that of nulliparous women.
      The use of a nulliparous comparison group is a strength of this study, as it allowed us to distinguish the change after pregnancy from any background age or secular trends in our population. Previous cross-sectional studies that compared nulliparous and parous women postpartum have generally found lower lipid levels among parous women after they deliver, consistent with our findings (
      • Natland S.T.
      • Nilsen T.I.L.
      • Midthjell K.
      • Andersen L.F.
      • Forsmo S.
      Lactation and cardiovascular risk factors in mothers in a population-based study: the HUNT-study.
      ,
      • Lv H.
      • Yang X.
      • Zhou Y.
      • Wu J.
      • Liu H.
      • Wang Y.
      • Pan Y.
      • Xia Y.
      Parity and serum lipid levels.
      ,
      • Hardy R.
      • Lawlor D.A.
      • Black S.
      • Wadsworth M.E.J.
      • Kuh D.
      Number of children and coronary heart disease risk factors in men and women from a British birth cohort.
      ). Our study suggests that the findings from these cross-sectional studies may have underestimated differences related to pregnancy, as nulliparous women had more atherogenic lipid profiles compared with parous women before their first birth. Given the less atherogenic lipid profiles among parous women earlier in life, it is striking that they experienced an almost identical TC/HDL-C ratio and a significantly worse HDL-C profile from postpartum through the age of 50 years.
      Our findings that breastfeeding may moderate the short-term adverse changes in HDL-C levels associated with pregnancy is consistent with findings from the CARDIA study (
      • Gunderson E.P.
      • Lewis C.E.
      • Wei G.S.
      • Whitmer R.A.
      • Quesenberry C.P.
      • Sidney S.
      Lactation and changes in maternal metabolic risk factors.
      ). However, the CARDIA study found more dramatic associations between breastfeeding and the change in HDL-C levels after birth, with those who breastfed 0 to <3 months having a −7.3 mg/dl drop in HDL-C compared with only a −1.3 mg/dl drop among women who breastfed ≥3 months. We were able to control for potential confounders not captured in the CARDIA study, including alcohol use, physical activity, and preterm first birth, and found that women who breastfed >6 months still experienced a drop in HDL-C of −2.9 mg/dl compared with a drop of −3.9 mg/dl among those who did not breastfeed. Despite controlling for several covariates, there is still the potential for confounding to explain some or all of the differences we observed across categories of breastfeeding length. In Norway, there are particularly high breastfeeding rates (
      • Lande B.
      • Andersen L.F.
      • Baerug A.
      • Trygg K.U.
      • Lund-Larsen K.
      • Veierød M.B.
      • Bjørneboe G.E.A.
      Infant feeding practices and associated factors in the first six months of life: the Norwegian infant nutrition survey.
      ) and fewer cultural barriers (
      Australian Government Department of Health and Ageing.
      ), potentially leading to less confounding by socioeconomic factors than in other contexts; however, the minority of women who do not breastfeed are still likely a highly selected population. While breastfeeding is associated with higher HDL-C levels before weaning (
      • Knopp R.H.
      • Walden C.E.
      • Wahl P.W.
      • Bergelin R.
      • Chapman M.
      • Irvine S.
      • Albers J.J.
      Effect of postpartum lactation on lipoprotein lipids and apoproteins.
      ,
      • Erkkola R.
      • Viikari J.
      • Irjala K.
      • Solakivi-Jaakkola T.
      One-year follow-up of lipoprotein metabolism after pregnancy.
      ,
      • Kallio M.J.
      • Siimes M.A.
      • Perheentupa J.
      • Salmenperä L.
      • Miettinen T.A.
      Serum cholesterol and lipoprotein concentrations in mothers during and after prolonged exclusive lactation.
      ,
      • Kjos S.L.
      • Henry O.
      • Lee R.M.
      • Buchanan T.A.
      • Mishell D.R.
      The effect of lactation on glucose and lipid metabolism in women with recent gestational diabetes.
      ) and a more rapid return of triglycerides to prepregnancy levels (
      • Darmady J.M.
      • Postle A.D.
      Lipid metabolism in pregnancy.
      ), it is also unclear whether these findings would extend over a longer period of time (
      • Natland S.T.
      • Nilsen T.I.L.
      • Midthjell K.
      • Andersen L.F.
      • Forsmo S.
      Lactation and cardiovascular risk factors in mothers in a population-based study: the HUNT-study.
      ,
      • Stuebe A.M.
      • Schwarz E.B.
      • Grewen K.
      • Rich-Edwards J.W.
      • Michels K.B.
      • Foster E.M.
      • Curhan G.
      • Forman J.
      Duration of lactation and incidence of maternal hypertension: a longitudinal cohort study.
      ). While we were able to quantify the association of breastfeeding duration with the change in lipid levels before and after pregnancy, we lacked sufficient data to examine long-term lipid trajectories by breastfeeding history.
      A limitation of this study is the lack of repeated measurements for all women. By including women who contributed only one measurement, we were able to examine a much larger span of time postpartum than had previous studies. However, such trajectories, constructed of both longitudinal and cross-sectional data, may not reflect an individual woman's trajectory across her life course, as there is the potential for secular trends to influence the shape of trajectories. Such concerns should not affect the comparisons of parous and nulliparous women, who experienced the same secular trends. We also found similar results after restricting our analysis to the 47% of women with repeated measurements, suggesting that it is reasonable to estimate within-woman changes from these trajectories.
      Another limitation of our study is the nonfasting collection of lipids. Although we controlled for the time since last meal in all analyses, we lacked information on the content of the last meal, which may have affected triglyceride measurements but likely had a minimal effect on other lipid measures (
      • Mora S.
      • Otvos J.D.
      • Rifai N.
      • Rosenson R.S.
      • Buring J.E.
      • Ridker P.M.
      Lipoprotein particle profiles by nuclear magnetic resonance compared with standard lipids and apolipoproteins in predicting incident cardiovascular disease in women.
      ,
      • Langsted A.
      • Freiberg J.J.
      • Nordestgaard B.G.
      Fasting and nonfasting lipid levels.
      ). This source of error combined with the relatively large variation in triglyceride measurements (
      • Mogadam M.
      • Ahmed S.W.
      • Mensch A.H.
      • Godwin I.D.
      Within-person fluctuations of serum cholesterol and lipoproteins.
      ) may have led to wider CIs for triglycerides compared with other lipid measures and could have obscured the effects of pregnancy. Of note, nonfasting lipids are predictive of CVD risk (
      • Langsted A.
      • Freiberg J.J.
      • Nordestgaard B.G.
      Fasting and nonfasting lipid levels.
      ,
      • Mora S.
      • Rifai N.
      • Buring J.E.
      • Ridker P.M.
      Fasting compared with nonfasting lipids and apolipoproteins for predicting incident cardiovascular events.
      ,
      • Nordestgaard B.G.
      • Benn M.
      • Schnohr P.
      • Tybjaerg-Hansen A.
      Nonfasting triglycerides and risk of myocardial infarction, ischemic heart disease, and death in men and women.
      ,
      • Bansal S.
      • Buring J.E.
      • Rifai N.
      • Mora S.
      • Sacks F.M.
      • Ridker P.M.
      Fasting compared with nonfasting triglycerides and risk of cardiovascular events in women.
      ), and thus their association with pregnancy is relevant for understanding early subclinical CVD risk. Although nonfasting lipid measurement limited us from accurately measuring LDL-C, non-HDL-C is considered as good as or a better predictor of CVD risk (
      • Ridker P.M.
      • Rifai N.
      • Cook N.R.
      • Bradwin G.
      • Buring J.E.
      Non-HDL cholesterol, apolipoproteins A-I and B100, standard lipid measures, lipid ratios, and CRP as risk factors for cardiovascular disease in women.
      ,
      • Liu J.
      • Sempos C.
      • Donahue R.P.
      • Dorn J.
      • Trevisan M.
      • Grundy S.M.
      Joint distribution of non-HDL and LDL cholesterol and coronary heart disease risk prediction among individuals with and without diabetes.
      ,
      • Cui Y.
      • Blumenthal R.S.
      • Flaws J.A.
      • Whiteman M.K.
      • Langenberg P.
      • Bachorik P.S.
      • Bush T.L.
      Non-high-density lipoprotein cholesterol level as a predictor of cardiovascular disease mortality.
      ). Lipid measurement methods also differed somewhat between HUNT2 and HUNT3 exams, which could have influenced the shape of our life-course lipid trajectories. Although we did not find evidence of systematically higher or lower lipid levels by HUNT exam, we controlled for HUNT survey occasion in all analyses, which would adjust our estimates of within-woman change for any differences in measurement methods.
      We also were unable to adjust for statin use, which may have altered the shape of lipid trajectories. However, rates of use were low among 20 to 60 year olds during our study period (
      • Hartz I.
      • Sakshaug S.
      • Furu K.
      • Engeland A.
      • Eggen A.E.
      • Njølstad I.
      • Skurtveit S.
      Aspects of statin prescribing in Norwegian counties with high, average and low statin consumption–an individual-level prescription database study.
      ), and usage is likely to have influenced the trend in the nulliparous and parous groups equally. Menopause transition or hormone replacement therapy use may also have affected lipid trajectories at older ages; however, controlling for these variables did not meaningfully change study findings in sensitivity analyses. We were also unable to control for either diet or sleep duration, which may change after pregnancy and might be considered one of the pathways through which parity affects lipid levels. However, a previous study did not find that diet explained a large portion of the drop in HDL-C after pregnancy (
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ). While our study location in Nord-Trøndelag county is fairly representative of Norway (
      • Holmen J.
      • Midthjell K.
      • Krüger Ø.
      • Langhammer A.
      • Holmen T.L.
      • Bratberg G.H.
      • Vatten L.
      • Lund-Larsen P.G.
      The Nord-Trøndelag Health Study 1995–97 (HUNT 2): Objectives, contents, methods and participation.
      ), the population is ethnically homogenous, which may limit the generalizability of our findings. Previous studies found similar differences in lipids associated with pregnancy among white and black women (
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ), suggesting that any effects of first birth on lipids are similar across a range of contexts.
      As with other studies on this topic, we found that the difference in HDL-C after pregnancy was still present after adjusting for changes in BMI and a variety of lifestyle factors postpartum (
      • Gunderson E.P.
      • Lewis C.E.
      • Murtaugh M.A.
      • Quesenberry C.P.
      • Smith West D.
      • Sidney S.
      Long-term plasma lipid changes associated with a first birth: the Coronary Artery Risk Development in Young Adults study.
      ,
      • Gunderson E.P.
      • Schreiber G.
      • Striegel-Moore R.
      • Hudes M.
      • Daniels S.
      • Biro F.M.
      • Crawford P.B.
      Pregnancy during adolescence has lasting adverse effects on blood lipids: a 10-year longitudinal study of black and white females.
      ,
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ,
      • Skilton M.R.
      • Bonnet F.
      • Begg L.M.
      • Juonala M.
      • Kähönen M.
      • Lehtimäki T.
      • Viikari J.S.A.
      • Raitakari O.T.
      Childbearing, child-rearing, cardiovascular risk factors, and progression of carotid intima-media thickness.
      ). The mechanism for the drop in HDL-C is only speculative, but one hypothesis is that it is related to hormonal changes postpartum. Parous women have tended to have lower levels of estrogen compared with nulliparous women in most, but not all, studies, including those extending over multiple years postpartum (
      • Barrett E.S.
      • Parlett L.E.
      • Windham G.C.
      • Swan S.H.
      Differences in ovarian hormones in relation to parity and time since last birth.
      ,
      • Toriola A.T.
      • Vääräsmäki M.
      • Lehtinen M.
      • Zeleniuch-Jacquotte A.
      • Lundin E.
      • Rodgers K-G.
      • Lakso H-A.
      • Chen T.
      • Schock H.
      • Hallmans G.
      Determinants of maternal sex steroids during the first half of pregnancy.
      ,
      • Arslan A.A.
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      • Lukanova A.
      • Afanasyeva Y.
      • Katz J.
      • Levitz M.
      • Del Priore G.
      • Toniolo P.
      Effects of parity on pregnancy hormonal profiles across ethnic groups with a diverse incidence of breast cancer.
      ,
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      ,
      • Dorgan J.F.
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      ,
      • Bernstein L.
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      Estrogen and sex hormone-binding globulin levels in nulliparous and parous women.
      ). Estrogen is known to be positively associated with HDL-C levels, suggesting estrogen suppression after pregnancy may be implicated. Declines in HDL-C with pregnancy have been shown to differ based on the apoE phenotype, suggesting an interaction between genetic and hormonal components (
      • Gunderson E.P.
      • Whitmer R.A.
      • Lewis C.E.
      • Quesenberry C.P.
      • West D.S.
      • Sidney S.
      Do long-term HDL-C declines associated with a first birth vary by apo E phenotype? The Coronary Artery Risk Development in Young Adults (CARDIA) study.
      ). In addition, studies have suggested that changes in HDL may differ by subclass, with larger HDL-2 particles declining after pregnancy more than other, smaller subclasses of HDL-C, leading to a redistribution toward a smaller particle size postpartum (
      • Lewis C.E.
      • Funkhouser E.
      • Raczynski J.M.
      • Sidney S.
      • Bild D.E.
      • Howard B.V.
      Adverse effect of pregnancy on high density lipoprotein (HDL) cholesterol in young adult women. The CARDIA Study. Coronary Artery Risk Development in Young Adults.
      ,
      • Zeljkovic A.
      • Vekic J.
      • Spasic S.
      • Jelic-Ivanovic Z.
      • Spasojevic-Kalimanovska V.
      • Gojkovic T.
      • Ardalic D.
      • Mandic-Markovic V.
      • Cerovic N.
      • Mikovic Z.
      Changes in LDL and HDL subclasses in normal pregnancy and associations with birth weight, birth length and head circumference.
      ). Only one previous study (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ) distinguished between changes in lipids after first and later births; this study found diminishing changes after subsequent births. Mankuta et al. (
      • Mankuta D.
      • Elami-Suzin M.
      • Elhayani A.
      • Vinker S.
      Lipid profile in consecutive pregnancies.
      ) also noted that the rise in HDL-C during pregnancy was smaller in each subsequent birth. Further studies to elucidate this mechanism are needed to explain why the postpregnancy drop in HDL-C depends on parity.
      These findings suggest that pregnancy is associated with a lasting adverse change in HDL-C that sets parous women on a more atherogenic trajectory than they had before pregnancy. Extrapolating from Hartz et al. (
      • Hartz I.
      • Sakshaug S.
      • Furu K.
      • Engeland A.
      • Eggen A.E.
      • Njølstad I.
      • Skurtveit S.
      Aspects of statin prescribing in Norwegian counties with high, average and low statin consumption–an individual-level prescription database study.
      ), the 4.2 mg/dl decrease in HDL-C with first birth would be associated with a 7% increase in the rate of coronary heart disease, while the 1.4 mg/dl decrease in HDL-C at the age of 50 years among parous women would be associated with a 2% increase in coronary heart disease rates, suggesting the short- and long-term changes in HDL-C associated with pregnancy are also associated with relatively small changes in CVD risk. While HDL-C is strongly predictive of CVD risk (
      • Gordon T.
      • Castelli W.P.
      • Hjortland M.C.
      • Kannel W.B.
      • Dawber T.R.
      High density lipoprotein as a protective factor against coronary heart disease. The Framingham Study.
      ,
      • Kannel W.B.
      High-density lipoproteins: epidemiologic profile and risks of coronary artery disease.
      ), recent findings suggest that raising HDL-C levels may not causally lead to improvements in cardiovascular endpoints (
      • Arslan A.A.
      • Zeleniuch-Jacquotte A.
      • Lukanova A.
      • Afanasyeva Y.
      • Katz J.
      • Levitz M.
      • Del Priore G.
      • Toniolo P.
      Effects of parity on pregnancy hormonal profiles across ethnic groups with a diverse incidence of breast cancer.
      ). Characterizing the effect of pregnancy on more direct measures of HDL function such as cholesterol efflux capacity (
      • Fisher E.A.
      • Feig J.E.
      • Hewing B.
      • Hazen S.L.
      • Smith J.D.
      HDL function, dysfunction, and reverse cholesterol transport.
      ,
      • Khera A.V.
      • Cuchel M.
      • de la Llera-Moya M.
      • Rodrigues A.
      • Burke M.F.
      • Jafri K.
      • French B.C.
      • Phillips J.A.
      • Mucksavage M.L.
      • Wilensky R.L.
      Cholesterol efflux capacity, high-density lipoprotein function, and atherosclerosis.
      ), the ability to remove cholesterol from cells, would be useful in future studies.

      Acknowledgments

      HUNT is a collaboration between the HUNT Research Centre (Faculty of Medicine and Health Sciences, Norwegian University of Science and Technology), Nord-Trøndelag County Council, Central Norway Regional Health Authority, and Norwegian Institute of Public Health.

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