Cohort composition
Of the 15,550,961 total live births in the US between 2015-2018, we sequentially excluded pregnancies to primiparous women and women of parity ≥3, multiple gestations, newborns with major malformations, missing gestational age, 886 with gestational age <22 weeks, and 120 with gestational age ≥45 weeks, as well as large for gestational age births. We additionally excluded pregnancies with missing data for TOL and previous caesarean delivery. After all exclusions, 3,755,798 non-anomalous, singleton live births delivered at 22-44 weeks remained in the cohort (eFigure 1 ).