Abstract Body: Background During pregnancy, systemic vascular resistance decreases, lowering blood pressure (BP) initially. Though BP typically returns to pre-pregnancy levels later in pregnancy, some evidence suggests postpartum BP may remain lower for years. However, long-term data into midlife, particularly by race, remains limited.
Objective To identify the association between childbirth and 35-year change in systolic (SBP) and diastolic (DBP) and the modifying effect of race.
Methods We included 804 women from the Bogalusa Heart Study (386 nulliparous, 418 parous at baseline), each with repeated measures of BP up to 13 times and reproductive history (Whites 63% and Blacks 37%). Women with baseline hypertension, hypertensive disorders of pregnancy, and hysterectomy were excluded. Cumulative number of births was modeled as a time-dependent exposure and categorized at each visit (0, 1, or ≥2 births) and updated at each follow-up visit. Outcomes were the changes in SBP and DBP from baseline, using BP measured only during non-pregnancy follow-up visits. Linear mixed effects models with random intercepts were used and adjusted for age, race, education, smoking, baseline BP, hypertension, oral contraceptive use, and weight changes with multiple imputations. Race-stratified analyses were conducted.
Results Among women nulliparous at baseline, 174 remained nulliparous, 57 had 1 birth, and 155 had ≥2 births. Compared to nulliparous women, SBP was 3.08 and 3.57 mmHg less with 1 and ≥2 births (SE=0.78, SE=0.73, p<0.001); DBP was 1.35 and 0.05 mmHg less (SE=0.58, SE=0.55, p=0.039). Among women parous at baseline, 20 had no additional births, 95 had 1, and 303 had ≥2. SBP was lower by 3.03 and 4.53 mmHg with 1 birth and ≥2 births (SE=0.74, SE=0.80, p<0.001); DBP was lower by 1.54 and 0.93 mmHg (SE=0.53, SE=0.57, p=0.009). Race modified associations (p _interaction<0.001 for SBP, 0.002 for DBP). Among White women nulliparous at baseline, SBP was lower by 3.57 and 5.49 mmHg with 1 birth and ≥ 2 births (SE=0.75, SE=0.85, p<0.001); DBP was lower by 1.76 and 1.52 mmHg (SE=0.56, SE=0.56, p=0.005). Among Black women, no BP changes were observed (all p>0.05). Similar racial patterns were found among women parous at baseline.
Conclusion Childbirth was associated with long-term BP reductions, especially among White women. These findings suggest that childbirth may reduce cardiovascular risk in midlife, and this role may vary by race, warranting further study.