Preterm induction of labor: predictors of vaginal delivery and labor curves

The purpose of this study was to evaluate the labor curves of patients who undergo preterm induction of labor (IOL) and to assess possible predictors of vaginal delivery (VD).

Data from the National Institute of Child Health and Human Development Consortium on Safe Labor were analyzed. A total of 6555 women who underwent medically indicated IOL at <37 weeks of gestation were included in this analysis. Patients were divided into 4 groups based on gestational age (GA): group A, 24-27+6 weeks; B, 28-30+6 weeks; C, 31-33+6 weeks; and D, 34-36+6 weeks. Pregnant women with a contraindication to VD, IOL ≥37 weeks of gestation, and without data from cervical examination on admission were excluded. Analysis of variance was used to assess differences between GA groups. Multiple logistic regression was used to assess predictors of VD. A repeated measures analysis was used to determine average labor curves.

Rates of vaginal live births increased with GA, from 35% (group A) to 76% (group D). Parous women (odds ratio, 6.78; 95% confidence interval, 6.38-7.21) and those with a favorable cervix at the start of IOL (odds ratio, 2.35; 95% confidence interval, 2.23-2.48) were more likely to deliver vaginally. Analysis of labor curves in nulliparous women showed shorter duration of labor with increasing GA; the active phase of labor was, however, similar across all GAs.

Most women who undergo medically indicated preterm IOL between 24 and 36+6 weeks of gestation deliver vaginally. The strongest predictor of VD was parity. Preterm IOL had a limited influence on estimated labor curves across GAs.