Age-period-cohort analysis of low birth weight, early term delivery, induction and csection, US births 1990-2019

We conducted an age-period-cohort (APC) analysis to distinguish age, period and cohort effects underlying temporal trends in: low birth weight (LBW: < 2500 grams), early term deliveries (ETD: 37-38 weeks), induction and c-section using US birth records from 1994-2019 for all singleton, first, full-term (37-44 weeks gestation) births (n = 8 194 523). Using the APC-interaction method, we estimated age, period and cohort effects as odds ratios with 95% confidence intervals (CI). Age and period effects were observed for all outcomes. Pregnant women/people 40-44 years had increased odds of LBW (OR 1.23, 95%CI 1.19-1.26), ETD (OR 1.24, 95%CI 1.23-1.26), induction (OR 1.05, 95%CI 1.04-1.06) and c-section (OR 2.28, 95%CI 2.26-2.30) compared to the total population. Odds of LBW (OR 1.14, 95%CI 1.13-1.16), ETD (OR 1.11, 95%CI 1.10-1.12) and induction (OR 1.71, 95%CI 1.70-1.72) were higher in 2019 compared to the study period average. Cohort effects not explained by age and period effects were observed for women 40-44 years in 2019 indicated by higher odds of ETD (OR 1.08, 95%CI 1.05-1.10), induction (1.10, 95%CI 1.07-1.12) and c-section (OR 1.09, 95%CI 1.06-1.11). These findings suggest obstetric management of pregnant women ≥ 40 years changed in recent decades in ways not expected based on age and temporal trends alone.