Association between timing of motherhood and prospective cardiovascular biomarker risk factors: a twin study

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.

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 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.

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.

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.

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.