Length of the second stage of labor and preterm delivery risk in the subsequent pregnancy

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Cervical injury is regarded as an important risk factor for preterm delivery. A prolonged second stage of labor may increase the risk of cervical injury that, in turn, may be associated with increased risk of spontaneous preterm delivery in the subsequent pregnancy. We sought to evaluate whether the duration of the second stage of labor in a term primiparous singleton delivery is associated with an increased risk of singleton spontaneous preterm delivery (<37 weeks) in the second pregnancy. We carried out a retrospective cohort analysis of women with 2 consecutive pregnancies: a first term (≥37 weeks) delivery and second birth. Data were derived from a single institution's prospectively collected obstetrical database from January 2005 through January 2015. Duration of the second stage of labor was examined as a continuous variable, modeled based on nonparametric restricted cubic regression spline with 4 degrees of freedom. Second-stage duration was also examined as short (<30 minutes), normal (30-179 minutes), and prolonged, defined as ≥180 minutes. The association between the duration of the second stage of labor in the first term pregnancy and the risk for spontaneous preterm delivery in the second pregnancy was evaluated before and after adjusting for potential confounders based on the Cox proportional hazards regression model. Associations were expressed based on the adjusted hazard ratio and 95% confidence interval. In all, 6715 women met inclusion criteria. The hazard of spontaneous preterm delivery in the second pregnancy trended higher with both shorter and longer second-stage labors. The length of the second stage of labor in the first term delivery was categorized as short (<30 minutes) in 1749 (26.0%), normal (30-179 minutes) in 4551 (67.8%), and prolonged (≥180 minutes), in 415 (6.2%) women. Of these 6715 women with a first term delivery, 4.2% (n= 279) delivered spontaneously preterm in the second pregnancy. The risks of spontaneous preterm delivery among women with prolonged (≥180 minutes) second stage of labor and normal labor duration (30-179 minutes) were 5.4% (n= 22) and 3.5% (n= 158), respectively (adjusted hazard ratio, 1.81; 95% confidence interval, 1.15-2.84). This increased risk for prolonged second stage of labor was primarily seen among women who underwent a cesarean (hazard ratio, 3.38; 95% confidence interval, 1.09-10.49), but was imprecise among women who delivered vaginally (hazard ratio, 1.52; 95% confidence interval, 0.62-3.74). The risk of spontaneous preterm delivery among women with short second stage of labor (<30 minutes) in their first term pregnancy was 5.8% (n= 99; hazard ratio, 1.28; 95% confidence interval, 0.99-1.67). The risk of spontaneous preterm delivery in the second pregnancy was increased in women with a prolonged (≥180 minutes) second stage in the first term pregnancy. This risk was even greater among women who were delivered by cesarean in the first pregnancy.