Abstract

Background: Evidence suggests that transitioning to motherhood at a younger age is associated with higher levels of cardiovascular biomarker risk factors later in life. While early-life confounding factors alongside social and behavioural pathways contribute to this association, residual confounding may remain. Objective: To investigate the relationship between age at first childbirth and later life cardiovascular biomarker risk factors (BMI, android/gynoid fat ratio, blood pressure, lipid profile), and environmental and genetic confounding in female twins. Participants and setting: Participants were 2,204 mothers from the TwinsUK cohort (549 di-, 553 monozygotic twin pairs) who were 50 years or older and had data on age at first birth, at least one outcome, and selected covariates. Methods: Generalised estimation equations were used to analyse (1) individual-level crude associations of age at first birth with the outcomes, (2) di- and monozygotic between and within-family estimates, and (3) covariate-adjusted associations. Results: Individual-level analyses suggest that women with age at first birth <20 years (compared to 25–29 years) had higher mean BMI, android/gynoid fat ratio, and triglyceride levels after age 50. However, confidence intervals were wide. Considering within-family estimates, effect size reductions suggest partial confounding by early environmental factors, with associations for android/gynoid fat ratio persisting. Conclusion: Family-level confounding plays a role in the link between age at first birth and cardiovascular biomarker risk factors. Age at first birth <20 may be associated with increased cardiovascular biomarker risk. Larger representative and/or twin studies are needed to assess these findings’ significance, robustness to confounding, and specific pathways.