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.