Association of Widespread Adoption of the 39-Week Rule With Overall Mortality Due to Stillbirth and Infant Death.

Importance: To improve neonatal morbidity, efforts have been made to reduce elective deliveries prior to 39 weeks' gestation, also known as the 39-week rule. Prolonging pregnancies also prolongs exposure to the risk of stillbirth. The true association of a 39-week rule with mortality is unknown and studies to date have shown conflicting results.

Objective: To determine if widespread adoption of a 39-week rule, limiting elective deliveries prior to 39 weeks' gestation, is associated with an increase or decrease in overall mortality when considering both stillbirths and infant deaths.

Design, Setting, and Participants: This historical cohort study used birth certificate and infant death certificate data in the United States to compare years before and after the adoption of the 39-week rule. Births between 2008 and 2009 were considered to be in the preadoption period (n = 7 322 234), and those between 2011 and 2012 were considered to be in the postadoption period (n = 6 972 626). Included births were singleton, nonanomalous births between 37 0/7 weeks' and 42 6/7 weeks' gestation. Statistical analysis was performed from July 19, 2016, through June 27, 2019.

Exposures: The exposure of interest was the Joint Commission adoption of the 39-week rule as a quality measure.

Main Outcomes and Measures: The primary outcomes of interest were stillbirth and infant death.

Results: A total of 7 322 234 births (49.0% girls and 51.0% boys) were included in the preadoption period and 6 972 626 births (49.1% girls and 50.9% boys) were included in the postadoption period. Compared with the preadoption period, there was a decrease in the proportion of deliveries at 37 weeks (-0.06%) and 38 weeks (-2.5%) and an increase in the proportion of deliveries at 39 weeks (6.8%) and 40 weeks (0.2%) in the postadoption period (P < .001). The stillbirth rate increased in the postadoption cohort compared with preadoption (0.09% vs 0.10%; P < .001). The infant death rate decreased in the postadoption period compared with preadoption (0.21% vs 0.20%; P < .001). An overall mortality rate of 0.31% was calculated for the preadoption period and 0.30% for the postadoption period (P = .06). Additional analysis in a counterfactual model suggests that up to 34.2% of the difference in mortality could be associated with the 39-week rule.

Conclusions and Relevance: Stable overall perinatal mortality rates were observed in the 2-year period immediately after adoption of the 39-week rule, despite an increase in stillbirth.