Study design and data sources
We performed a cross-sectional analysis of all singleton births in the US from 2015-2018. We utilized data on fetal deaths, live births, and linked live births-infant death records ascertained by the National Center for Health Statistics (NCHS) of Centers for Disease Control and Prevention. The live birth-infant death data is derived from infant death certificates linked to the live birth certificates for infants that died before reaching 1 year of age. Study variables were derived from the 2003 revision of the US standard certificates of vital events. Gestational age in these data were based on the best obstetrical estimate,(27) which had a 99.6% agreement within 2 weeks in comparison to data abstracted from hospital charts.(28) Since this de-identified data is publicly available, no ethics approval from an Institutional Review Board was sought. The study followed the STROBE reporting guidelines for cross-sectional studies.