Long-term outcomes according to birth weight after full-term pregnancy: a nationwide cohort study.

Birth weight (BW) is a key determinant of neonatal health, with deviations from the normal range potentially influencing long-term outcomes. We examined the association between BW and long-term outcomes in neonates based on BW after full-term pregnancy. This retrospective observational cohort study analyzed data from the Korea National Health Insurance claims database. Long-term adverse outcomes, including neurological or neurodevelopmental outcomes, were assessed based on prespecified diagnoses, with follow-ups on all neonates. In total, 514,166 mother-infant pairs were included in the study. The rates of neurological or neurodevelopmental delay in the groups with BWs between the 50 and 75th percentiles (controls), below the 3rd percentile, and above the 97th percentile were 14.5, 17.9, and 16.0%, respectively. Compared with the control group, the groups with BWs below the 3rd percentile and in the 3-5, 5-10, 95-97, and > 97th percentiles had adjusted hazard ratios of 1.24 (95% confidence interval: 1.20-1.28), 1.10 (1.05-1.15), 1.10 (1.06-1.13), 1.05 (0.98-1.12), and 1.09 (1.03-1.15), respectively. The relationship between neonatal BW and neurological or neurodevelopmental delays followed a J-shaped distribution, indicating that infants at both extremes of BW were at an increased risk for these delays.