Neonatal complications of term pregnancy: Rates by gestational age increase in a continuous, not threshold, fashion

The purpose of this study was to determine whether, when, and how rates of short-term neonatal complications increase beyond 37 weeks of gestation. A retrospective cohort study was conducted of all low-risk, term, cephalic, and singleton births that were delivered at the University of California, San Francisco, between 1976 and 2001. Primary outcomes included neonatal umbilical artery pH, umbilical artery base excess, the presence of meconium, macrosomia, 5-minute Apgar scores, and admission to the intensive care nursery. Multivariate analyses were performed that controlled for maternal ethnicity, weight, age, socioeconomic status, and obstetric history. Among the 32,679 women who were delivered at ≥37 completed weeks of gestation, the rates of umbilical artery pH <7.0, umbilical artery base excess less than -12 increased beyond 40 weeks of gestation, and the presence of meconium increased beyond 39 weeks of gestation (chi-squared test; P <. 001). These outcomes continued to increase in each subsequent week, and these findings persisted when they were controlled for potential confounders in multivariate models. We found that the rates of immediate neonatal morbidity increase with increasing gestational age. Accurate determination of these rates is important in the determination of gestational age at which the risk of continuing the pregnancy outweighs the risk of induction of labor.