Objectives: To evaluate the association between antenatal betamethasone exposure and neonatal respiratory morbidity among late-preterm births. We further examined whether gestational age at delivery and the exposure-to-delivery interval modify this association. Methods: We conducted a retrospective cohort study of singleton live births at 34–36 + 6 weeks in a tertiary center (2011–2023). Betamethasone exposure was classified as none, early (7 days and evaluated separately at 34, 35, and 36 weeks. Primary outcomes were RDS and composite respiratory morbidity (RDS, TTN, or ≥3 days of respiratory support); neonatal hypoglycemia was secondary. Adjusted odds ratios were estimated using multivariable logistic regression including maternal age, parity, delivery mode, and birthweight. Results: The study included 2668 late-preterm infants, of whom 2356 (88.3%) were unexposed and 312 (11.7%) were exposed to antenatal corticosteroids (ACSs). Among exposed pregnancies, 138 (44.2%) received early ACS and 174 (55.8%) late ACS; 163 (52.2%) delivered ≤7 days and 149 (47.8%) >7 days after administration. Late ACS exposure was associated with lower odds of RDS (aOR 0.37, 95% CI 0.17–0.69) and composite respiratory morbidity (aOR 0.55, 95% CI 0.31–0.92), but with increased odds of neonatal hypoglycemia (aOR 2.72, 95% CI 1.26–5.31). Among pregnancies delivering at 34 weeks, exposure within ≤7 days was associated with a marked reduction in RDS (aOR 0.25, 95% CI 0.07–0.79; NNT ≈ 3), whereas no respiratory benefit was observed at 35 or 36 weeks or when the interval exceeded 7 days. Conclusions: Antenatal betamethasone exposure among late-preterm births was not uniformly associated with neonatal respiratory outcomes, with associations varying by gestational age at delivery and the exposure-to-delivery interval. These findings may be interpreted in the context of potential respiratory benefit alongside accompanying metabolic risk, with exploratory analyses suggesting a more pronounced signal among deliveries at 34 weeks within ≤7 days.

Background: Parity is associated with differences in birthweight, but the magnitude and consistency of this association when expressed as birthweight percentiles remain incompletely characterized. Objective: To evaluate the association between birth order and birthweight percentile at both the population-level and within mothers across successive pregnancies. Methods: This retrospective cohort study included 130,718 singleton live births from 55,057 women delivering at a single tertiary medical center between 2012 and 2024. Birthweight percentiles were calculated using a national sex-specific reference. Linear mixed-effects models with a random intercept for mother were used to evaluate the association between birth order and birthweight percentile, adjusting for maternal age, infant sex, and maternal diabetes. Complementary longitudinal within-mother analyses evaluated changes in birthweight percentile across successive pregnancies. Results: Higher birth order was associated with higher birthweight percentile ( β ≈ 0.074 SD per birth order increment, p < 0.001), corresponding to an average increase of approximately 3 percentile points per birth. The largest increase occurred between the first and second births, followed by progressively smaller but persistent increases with advancing birth order. This pattern was consistent across subgroup analyses and complementary within-mother analyses. Conclusions: Higher birth order was consistently associated with higher birthweight percentile both across the population and within mothers. The association followed a distinct nonlinear pattern, with the largest increase occurring between the first and second pregnancies and progressively smaller but persistent increases thereafter. These findings provide a more nuanced understanding of parity-associated variation in fetal growth across successive pregnancies.