Effect of late-preterm birth and maternal medical conditions on newborn morbidity risk

Late-preterm infants (34-36 weeks' gestation) account for nearly three quarters of all preterm births in the United States, yet little is known about their morbidity risk. We compared late-preterm and term (37-41 weeks' gestation) infants with and without selected maternal medical conditions and assessed the independent and joint effects of these exposures on newborn morbidity risk.

We used 1998-2003, population-based, Massachusetts birth and death certificates data linked to infant and maternal hospital discharge records from the Massachusetts Pregnancy to Early Life Longitudinal data system. Newborn morbidity risks that were associated with gestational age and selected maternal medical conditions, both independently and as joint exposures, were estimated by calculating adjusted risk ratios. A new measure of newborn morbidity that was based on hospital discharge diagnostic codes, hospitalization duration, and transfer status was created to define newborns with and without life-threatening conditions. Eight selected maternal medical conditions were assessed (hypertensive disorders of pregnancy, diabetes, antepartum hemorrhage, lung disease, infection, cardiac disease, renal disease, and genital herpes) in relation to newborn morbidity.

Our final study population included 26,170 infants born late preterm and 377,638 born at term. Late-preterm infants were 7 times more likely to have newborn morbidity than term infants (22% vs 3%). The newborn morbidity rate doubled in infants for each gestational week earlier than 38 weeks. Late-preterm infants who were born to mothers with any of the maternal conditions assessed were at higher risk for newborn morbidity compared with similarly exposed term infants. Late-preterm infants who were exposed to antepartum hemorrhage and hypertensive disorders of pregnancy were especially vulnerable.

Late-preterm birth and, to a lesser extent, maternal medical conditions are each independent risk factors for newborn morbidity. Combined, these 2 factors greatly increased the risk for newborn morbidity compared with term infants who were born without exposure to these risks.