Differences in Risk Factors for Recurrent Versus Incident Preterm Delivery

Risk factors for preterm delivery have been described, but whether risk factors differ in the context of prior preterm delivery history is less understood. We assessed whether known risk factors were different in women with versus without prior preterm delivery using medical records of the first and second singleton deliveries in 25,820 Utah women (2002-2010). Longitudinal transition models with modified Poisson regression calculated adjusted relative risks and 95% confidence intervals, with multiplicative interactions between each preterm risk factor and prior preterm delivery status to explore whether risk factors varied between incident and recurrent preterm delivery at <37 weeks. Fewer second pregnancy factors were associated with recurrent preterm delivery, including alcohol, thyroid disease, and depression. Smoking was associated with increased risk for incident (relative risk (RR) = 1.95, 95% confidence interval (CI): 1.53, 2.49) but not recurrent (RR = 1.09, 95% CI: 0.71, 1.19) preterm delivery, whereas alcohol was associated with an increased risk for recurrent (RR = 2.38, 95% CI: 1.53, 3.71) but not incident (RR = 0.98, 95% CI: 0.67, 1.43; Pinteraction = 0.02 and <0.01) preterm delivery, respectively. Prior term delivery did not necessarily confer protection from known second pregnancy preterm delivery risk factors. In the setting of a prior preterm delivery, many risk factors did not persist. Prior preterm delivery history is important when assessing subsequent preterm delivery risk factors.