Pre-induction sonographic measurement of cervical length in prolonged pregnancy: the effect of parity in the prediction of induction-to-delivery interval.

OBJECTIVE To examine the effect of parity on the relationship between pre-induction cervical length and the induction-to-delivery interval and rate of vaginal delivery within 24 h in women undergoing induction of labor for prolonged pregnancy. METHODS In 382 singleton pregnancies, induction of labor was carried out at 41 + 3 to 42 + 1 weeks of gestation. The cervical length was measured by transvaginal sonography before induction. Univariate analyses were performed by constructing Kaplan-Meier survival curves for the induction-to-delivery interval for various subgroups, and comparing these using log rank tests. Multivariate analyses were performed using the Cox proportional hazards model and multiple linear regression. RESULTS Successful vaginal delivery within 24 h of induction occurred in 67% of the women and the pre-induction cervical length was significantly associated with the induction-to-delivery interval and the rate of vaginal delivery within 24 h. Sonographically measured cervical length was better than the Bishop score or cervical length by vaginal examination in predicting the outcome of induction. Parity provided a significant independent contribution, in addition to pre-induction cervical length, in the prediction of the outcome of labor. Thus, in multiparae the incidence of successful vaginal delivery within 24 h of induction was about 30% higher than in nulliparae. For the same cervical length, the induction-to-delivery interval in multiparae was 37% lower than in nulliparae. CONCLUSION In women undergoing induction of labor for prolonged pregnancy, cervical length and parity provide independent prediction of induction-to-delivery interval and the likelihood of vaginal delivery within 24 h of induction.