Maternal factors and risk of late preterm delivery

Background: Late preterm infants(LP, 34-36 wks gestation) account for 70% of all premature infants. Little is known about why the birth rate in this group is rising. Objective: To determine if there are maternal risk factors associated with delivery of LP. Methods: Data were obtained from NY Vital Statistics on LP and term(³37 wks gestation) infants born in Westchester County, NY from 2004-2005. Poisson regression was used for analysis of relative risks(RR). Results: There were 2320 LP births and 20936 term births with a total birth population of 25165. There was an increased risk of c-section (c/s) delivery in LP compared to terms(RR1.05, 95% CI 1.02-1.07). There was no difference in the risk of having an elective c/s between LP and terms(RR1.11, 95% CI 0.97-1.27). The risk of having a c/s for maternal conditions related to pregnancy or fetal risk was increased in LP vs. terms(RR1.55, 95% CI 1.39-1.72; RR1.16, 95% CI 1.08-1.24). There was an increased risk of LP delivery in mothers <17 and >35 yrs old(RR1.14, 95% CI 1.02-1.28; RR1.04, 95% CI 1.02-1.06). Conclusions: LP are more likely than terms to be delivered via c/s for maternal conditions related to pregnancy and fetal risk. They are more likely to be born to mothers <17 and >35 yrs old. There was no difference in risk of elective c/s in LP vs. term infants suggesting an unexpectedly high rate of elective preterm delivery. This study allows us to begin to identify and target mothers at risk for LP delivery.