Study design and data sources
We performed a cross-sectional analysis of non-anomalous, singleton live births who were delivered in the US from 2015-2018 using natality data assembled by the National Center for Health Statistics (NCHS). These data are collected by delivery attendants and transferred to the NCHS where they are cleaned, compiled, and de-identified. Demographic characteristics, medical and health information, and newborn outcomes were derived from the 2003 revision of the US standard certificate of live birth. All data used in this study are publicly available in de-identified form, and Institutional Review Board approval was therefore not sought. The study followed the STROBE reporting guidelines for cross-sectional studies.
The study population included women who delivered non-anomalous, singleton live births between 22-44 weeks’ gestation. We restricted the analysis to para 2 women to avoid confounding by number of prior caesarean deliveries.