Recent studies have shown that the cause of very preterm births may be related to neonatal morbidity and mortality. Even though these risks are lower among late preterm births, this group accounts for the vast majority of all preterm births. The objective of this study was to evaluate the relation of neonatal morbidity and mortality to the cause of late preterm birth.

This retrospective observational cohort study included all women who gave birth to liveborn singletons from 34 to 36 weeks+6 days of gestation in a French level III maternity hospital in the 5-year period 2013–2017. The causes of preterm delivery were divided into 6 mutually exclusive groups. The main outcome was a composite neonatal morbidity criterion, defined by at least one among the following criteria: neonatal respiratory distress, neurological complications, neonatal sepsis, severe necrotizing enterocolitis, and neonatal hypoglycemia. We analyzed the association between cause of preterm delivery and neonatal morbidity after adjustment for gestational age and antenatal corticosteroid therapy. The reference group was preterm labor, defined by spontaneous preterm labor with intact membranes.

During the study period, there were a total of 27 110 births, including 1114 singleton births at 34 to 36 weeks of gestation + 6 days (4.1%). Among the 968 late preterm births included, the risk of neonatal morbidity in the group with preterm premature rupture of membranes (PPROM) was similar to that in the preterm labor (reference) group: adjusted odds ratio (aOR) 1.2 (95% CI, 0.8-1.8). All the other causes of late preterm birth were associated with a higher risk of neonatal morbidity than the reference group: aOR 2.0 [95% CI, 1.1–3.5] for hypertensive disorders without suspected fetal growth restriction (FGR) (9.1% of cases), aOR 2.4 [95% CI, 1.4–4.2] for hypertensive disorders with suspected FGR (8.9%), aOR 4.2 [95% CI, 2.2-8.0] for suspected FGR without hypertensive disorders (5.8%), and aOR 4.4 [95% CI, 2.2-8.8] for vaginal bleeding related to abnormal placental insertion (4.7%).

Among infants born from 34 to 36 weeks + 6 days of gestation, PPROM and preterm labor had similar risks of neonatal morbidity, while the other causes were associated with a risk of neonatal morbidity at least twice that with preterm labor.