Is Parity a Risk Factor for Late Preterm Birth? Results from a Large Cohort Study.

Most preterm births occur in the late preterm period. While prematurity-related adverse outcomes are significantly diminished when birth occurs during this period, these infants are still at increased risk of complications. Parity affects the incidence of obstetric complications. The purpose of this study was to determine whether parity impacts the risk of spontaneous late preterm birth (SLPTB) and associated complications. A retrospective observational cohort study was conducted. Patients were divided into three study groups according to parity. The primary outcome was the rate of SLPTB in each group. Secondary outcomes were unplanned cesarean delivery (UCD), prolonged third stage of labor respiratory distress syndrome (RDS), transient tachypnea of the newborn (TTN), intraventricular hemorrhage (IVH), neonatal hypoglycemia, duration of NICU admission, neonatal death, and composite adverse neonatal outcome (CANO). Primiparas were more likely to have SLPTB, UCD, and CANO compared to multiparas (2.6% vs. 1.9% OR 1.5 [1.3-1.7] p < 0.01) (4.1% vs. 1.3% OR 2.7 [1.2, 5.9] p < 0.01) (8.5% vs. 4.2 OR 2.1 [1.3-3.5] p = 0.002) and grandmultiparas (2.6% vs. 1.7% OR 1.4 [1.2-1.5] p < 0.001) 8.5% vs. 4.4% OR 2.0 [1.1, 3.8], p = 0.01) but no difference in UCD compared to grandmultiparas (4.1% vs. 3.3% OR 1.2 [0.6-2.7] p = 0.28). Primiparas are at increased risk of SLPTB and UCD, and this is accompanied by an increased risk of adverse neonatal outcomes.