Association of Temporal Changes in Gestational Age With Perinatal Mortality in the United States, 2007-2015

Importance Whether the changing gestational age distribution in the United States since 2005 has affected perinatal mortality remains unknown. Objective To examine changes in gestational age distribution and gestational age–specific perinatal mortality. Design, Setting, and Participants This retrospective cohort study examined trends in US perinatal mortality by linking live birth and infant death data among more than 35 million singleton births from January 1, 2007, through December 31, 2015. Exposures Year of birth and changes in gestational age distribution. Main Outcomes and Measures Changes in the proportion of births at gestational ages 20 to 27, 28 to 31, 32 to 33, 34 to 36, 37 to 38, 39 to 40, 41, and 42 to 44 weeks; changes in perinatal mortality (stillbirth at ≥20 weeks, and neonatal deaths at Results Among the 34 236 577 singleton live births during the study period, the proportion of births at all gestational ages declined, except at 39 to 40 weeks, which increased (54.5% in 2007 to 60.2% in 2015). Overall perinatal mortality declined from 9.0 to 8.6 per 1000 births ( P P P Conclusions and Relevance Although the proportion of births at gestational age 39 to 40 weeks increased, perinatal mortality at this gestational age declined. This finding may be owing to pregnancies delivered at 39 to 40 weeks that previously would have been unnecessarily delivered earlier, leaving fetuses at higher risk for mortality at other gestational ages.