Late-preterm infants, born at 34 to 36 weeks’ gestation, make up nearly three-fourths of all preterm births in the United States. The proportion of such infants among singleton live births has risen by more than 10% in the past decade, largely because of more frequent obstetrical interventions for maternal complications or preexisting medical disorders. This study compared newborn morbidity risk in late-preterm and term (37–41 weeks’ gestation) infants born in Massachusetts in the years 1998–2003. Infant and maternal hospital discharge records were taken from the Massachusetts Pregnancy to Early Life Longitudinal data system. Eight maternal medical conditions were analyzed for their effects on newborn morbidity: hypertensive disorders of pregnancy, diabetes, antepartum hemorrhage, lung disease, infection, cardiac disease, renal disease, and genital herpes. The final study population included 26,170 late-preterm infants and 377,638 born at term.
Late-preterm infants were 7 times more likely than term infants to have morbidity at birth (22.2% vs. 3.0%). Morbidity rates approximately doubled for each gestational week earlier than 38 weeks. Compared with infants born at 40 weeks’ gestation, those born at 34 weeks’ gestation had a 20-fold increase in risk, and infants born at 35 and 36 weeks’ gestation had, respectively, 10 times and 5 times the morbidity risk. Among both term and late-preterm infants, the incidence of morbidity was higher in males and those whose mothers were 40 or more years of age, were primiparas or grand multiparas, or smoked while pregnant. Late-preterm infants whose mothers had any of the 8 specified disorders were at higher risk for newborn morbidity than were similarly exposed near term or term infants. Infants exposed to antepartum hemorrhage or hypertensive disorders of pregnancy were especially at risk. The proportion of infants with morbidity increased with the number of maternal morbid conditions.
Both late-preterm birth and maternal medical disorders are independent risk factors for newborn morbidity and, when combined, greatly increase the risk of newborn morbidity compared with infants escaping exposure to either of these risks.