Association between birth order and birthweight percentile: population-level and within-mother analyses in a large birth cohort

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.