Our study findings show that both Black and White parous adolescents experienced greater decrements in HDL-C (4.3 to 4.5 mg/dl) and greater increments in fasting triglycerides (10.4 to 11.6 mg/dl) after pregnancy compared with lipid changes for nulligravid adolescents during the same 10-year study period. These differences in HDL-C and triglycerides for parous adolescents remained significant after adjustment for BMI and lipid measurements at age 9–10 years, age at menarche, family socio-demographics, and lifestyle behaviors, except for triglycerides in White adolescents. The strength of these associations was similar among Black and White adolescents, despite fewer pregnancies in Whites.
Excess weight gain and increased use of hormonal contraception after pregnancy appeared to modestly mediate the association between adolescent pregnancy and pro-atherogenic lipid profiles, primarily for attenuation of differences in HDL-C among the Black adolescents. Yet, both HDL-C and triglyceride differences among parous versus nulligravid participants remained statistically significant after controlling for overall adiposity gains, except for triglyceride differences among Whites. Our sensitivity analysis, in which we excluded hormonal contraceptive users, showed that our findings remained robust among participants never using hormonal contraceptives.
Our mean concentrations for LDL-C and total cholesterol in the NGHS are comparable to national estimates for LDL-C and total cholesterol, respectively, in adolescent females aged 12–17 years; 93.5 mg/dL and 165.9 mg/dL for black, and 89.8 mg/dL and 165.4 mg/dL for white.28
Another longitudinal study of lipid changes in females (20% Black) followed from age 9 to 18 years reported total cholesterol decrements of 19 mg/dl and triglycerides increments of 15 mg/dl, but did not report whether the adolescents had given birth.13
The lipid changes observed within the NGHS cohort are consistent with the previous study, particularly for the White females, but NGHS Black females showed modest decrements in fasting triglycerides.
Our finding that a first birth is associated with lower mean HDL-C independent of weight gain among adolescents is consistent with previous findings for adult women. CARDIA reported that primiparity compared with nulligravidity was associated with 3 to 4 mg/dl lower plasma HDL-C within 2 to 8 years after delivery among both Black women and White women aged 20–31 years.6;7
The HDL-C decrements in CARDIA women persisted after the first birth, and were not greater with subsequent births controlling for changes in adiposity and lifestyle behaviors (i.e., physical activity, alcohol intake).7
In contrast to CARDIA findings, parous adolescents in NGHS also showed higher fasting triglycerides than nulligravidas, although statistical significance was not reached for White adolescents possibly due to the smaller sample of parous adolescents. Implications of our findings are that a more atherogenic lipid profile at younger ages could influence the long-term risk of cardiometabolic diseases in adulthood, 12
and fetal programming in future pregnancies.29
Lower HDL-C levels associated with primiparity represent a 6 to 12 percent greater risk of CVD during midlife.30
Higher parity also has been directly associated with greater risk of CVD in older women, although residual confounding remains an issue.3;31
Black–white differences in physical maturation and the overall pattern of adolescent growth are well known.22;32
Black females reach menarche earlier, and have greater peak velocities in growth, followed slower growth in late adolescence than White females.22
Pregnant adolescents tend to accrue more subcutaneous fat in central locations compared with adult women,33;34
particularly younger, growing pregnant adolescents.18
Previously, paras compared to nulligravid NGHS adolescents had greater increases in both overall and central adiposity.20
However, weight gain did not explain the lower HDL-C or greater triglycerides among paras in our analysis. The specific mechanism for the lipid changes is unclear, but insulin resistance does not explain our findings because the lower HDL-C and higher TG remained after controlling for adiposity. Moreover, parity is not associated with increased incidence of type 2 diabetes after pregnancy in longitudinal studies,35;36
except among women with a history of gestational diabetes mellitus (GDM),36
which is uncommon in females <20 years of age.
Limitations include fewer White than Black parous adolescents, variable ages for deliveries, later maturation of White girls, and the tendency for Black adolescents to become pregnant at younger ages. We also did not have sufficient numbers of multiparas to assess whether decrements in HDL-C showed a threshold effect or a monotonic trend with higher order births. We adjusted for age at menarche, baseline lipid measurements, and socio-demographic covariates to minimize these differences, but they may still be influential. Hormonal contraceptive use during follow-up was a consequence of prior pregnancies, and appeared to mediate rather than confound our findings. Although we did not assess blood glucose and insulin in our models, adjustment for baseline BMI and changes in BMI during the 10-year period accounted for these metabolic characteristics which may result from excess fat deposition. Adolescents may have under-reported pregnancies ending in miscarriage or abortions which would bias our findings toward the null hypothesis.
The study strengths include the large, community-based sample of Black and White girls that provides an internal comparison group of never pregnant adolescents to evaluate the direct effects of pregnancy on adolescent blood lipid profiles independent of growth in stature, maturation (age at menarche), and secular trends. Blood lipid measurements were obtained prospectively via standard research methodology both before and after pregnancies.
Our findings are potentially important because adolescence has been identified as one of the “critical periods” of growth and development that set the stage for future adult chronic disease, including diabetes and cardiovascular diseases.37
Excessive fat deposition during adolescence may lead to persistent obesity,38
elevated insulin, atherogenic lipids and higher blood pressure levels into young adulthood.39
Relevant to our findings, HDL-C and triglycerides are important predictors of future cardiovascular disease and possibly, diabetes in adulthood.40
Further, in Black women, earlier age at a first birth (<20 years) has been associated with increasing rates of coronary heart disease.31;41
Our findings show that pregnancy at an early age results in lowering of HDL-C and raising of triglycerides that is not explained by pregnancy-related weight retention. Pregnancy during adolescence may have even greater adverse effects on women’s future cardiometabolic health in mid life.