LAUSR

Objective: To examine how changes in induction of labor (IOL) and cesarean deliveries between 1990 and 2017 affected gestational age distributions of births in the United States. Materials and Methods: Singleton first births were drawn from the National Vital Statistics System Birth Data for years 1990-2017. Separate analytic samples were created (1) by maternal race/ethnicity (Hispanic, non-Hispanic Black, non-Hispanic Asian, and non-Hispanic white), (2) by maternal age (15-19, 20-24, 25-29, 30-34, 35-39, 40-49), (3) by U.S. states, and (4) for women at low risk for obstetric interventions (e.g., age 20-34, no hypertension, no diabetes, no tobacco use). Gestational age was measured in weeks, and obstetric intervention status was measured as: (1) no IOL, vaginal delivery; (2) no IOL, cesarean delivery; and (3) IOL, all deliveries. The joint probabilities of birth at each gestational week by obstetric intervention status for years 1990-1991, 1998-1999, 2007-2008, and 2016-2017 were estimated. Results: Between 1990 and 2017, the percent of singleton first births occurring between 37 and 39 weeks of gestation increased from 38.5% to 49.5%. The changes were driven by increases in IOL and a shift in the use of cesarean deliveries toward earlier gestations. The changes were observed among all racial/ethnic groups and all maternal ages, and across all U.S. states. The same changes were also observed among U.S. women at low risk for interventions. Conclusion: Changes in gestational age distributions of U.S. births and their underlying causes are likely national-level phenomena and do not appear to be responding to increases in maternal risk for interventions.