Increases in Obstetric Interventions and Changes in Gestational Age Distributions of U.S. Births.

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. 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 ( 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. 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. 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.