Bias in comparisons of mortality among very preterm births: A cohort study

BACKGROUND: Several studies of prenatal determinants and neonatal morbidity and mortality among very preterm births have resulted in unexpected and paradoxical findings. We aimed to compare perinatal death rates among cohorts of very preterm births (24-31 weeks) with rates among all births in these groups (≥24 weeks), using births-based and fetuses-at-risk formulations. METHODS: We conducted a cohort study of singleton live births and …