Exposure
The exposure was SGA, defined as sex-specific birthweight <5th and <3rdpercentiles for gestational age based on a previously established US birthweight reference for singleton live births (13). We excluded large for gestational age births (i.e., 90th percentile or higher) since these fetuses may have different thresholds for labour tolerance and caesarean delivery risks compared to SGA births. Infants with birthweight-for-gestational age between the 10-89th percentiles constituted the appropriate for gestational age (AGA) reference group.