Re: “Differences in Risk Factors for Recurrent Versus Incident Preterm Delivery”

Preterm delivery at <37 weeks of gestation complicated 11.5% of US births in 2012 (1). Prior research has established maternal risk factors for preterm delivery including non-Hispanic black race, lean body mass index (weight (kg)/height (m)2) of <19.8, and smoking (2–5). One of the major risk factors for preterm delivery is a history of prior preterm delivery, which is known to increase the risk of subsequent preterm delivery from 1.5- to 2-fold (6, 7). Yet preterm delivery also occurs in 5%–9% of deliveries for women without a prior preterm delivery, and less attention has been given to this particular group of women (7, 8). Understanding the relationship of risk factors for preterm delivery in a current pregnancy in the context of prior preterm delivery history is important because there may be behavioral or clinical markers that can help predict risk in a second pregnancy. For example, women with a prior term birth may or may not believe that they are susceptible to a risk factor for preterm birth in their second pregnancy and vice versa for women with a prior preterm delivery. Risk and patterns of recurrence for preterm delivery are known to differ on the basis of prior subtype, spontaneous or indicated, so it is important to understand whether risk factors might vary according to the prior preterm delivery subtype (9). Our objectives were to 1) explore whether risk factors for preterm delivery in a second pregnancy differed on the basis of whether the first pregnancy delivered at term (≥37 weeks of gestation) or preterm (<37 weeks), 2) explore how changes in behavioral risk factors between pregnancies were associated with preterm delivery in the context of prior history, and 3) investigate whether risk factors for recurrent preterm delivery differed after taking prior subtype of preterm delivery, spontaneous or indicated, into account.