Obstetric characteristics and pregnancy outcomes under China's three-child policy: a retrospective cohort study

Background This retrospective cohort study aims to evaluate the association between maternal age, parity, and prior cesarean section (CS) with adverse perinatal outcomes under China's three-child policy and to assess whether the combination of advanced age, multiparity, and prior CS modifies these associations. Methods Data were extracted from the monitoring information management system of 14 obstetric hospitals in Fujian Province between 1 January and 31 December 2023. A total of 27,002 deliveries were included. Maternal age was categorized as 20–34, 35–39, and ≥40 years. Parity was classified as primipara vs. multipara and prior CS as yes vs. no. Adjusted odds ratios (aOR) with 95% confidence intervals (CI) were calculated. Results Among 27,002 women, advanced maternal age (≥40 years) alone was significantly associated with increased odds of gestational diabetes mellitus (GDM) (aOR = 1.98, 95% CI: 1.69–2.30) and hypertensive disorders of pregnancy (HDP) (aOR = 2.27, 95% CI: 1.65–3.11). The combination of advanced age, multiparity, and prior CS (three factors) was significantly associated only with anemia (aOR = 2.79, 95% CI: 2.39–3.27). No significant associations were observed for GDM, placenta previa, preterm birth, macrosomia, or low Apgar scores in the three-factor group. The lower odds of several outcomes (e.g., overall pregnancy complications and HDP) among multiparous women and those with prior cesarean section likely reflect selection bias. Conclusions In the context of China's three-child policy, advanced maternal age remains a strong independent risk factor for GDM and HDP. However, the combination of advanced age, multiparity, and prior CS significantly increases only the risk of anemia. The apparent inverse associations for other outcomes are best explained by preferential childbearing among healthier women. Early identification of anemia and targeted interventions for high-risk pregnancies are essential for improving maternal and child health.