Cervical score and onset of spontaneous labor in prolonged pregnancy dated by second-trimester ultrasonic scan.

The interval from expected day of delivery to spontaneous onset of labor was correlated with parity and cervical score in 103 women with uncomplicated prolonged pregnancy (greater than 294 days). All women had a routine ultrasonic scan in weeks 16-18 for the purpose of dating. The mean (+/- SD) modified Bishop score on entry to the study was 4.15 +/- 2.0 for nulliparas and 4.90 +/- 2.1 for multiparas. The duration beyond 294 days to spontaneous onset of labor varied little (mean 3.5-4.5 days) for nulliparas with scores greater than 2 and for multiparas regardless of score. Nulliparous women with a poor score (less than 3) had spontaneous onset of labor and delivery within a mean of 9.8 days. Half of the multiparas (50.0%) and 43.9% of the nulliparas gave birth within 3 days. About 90% of all women gave birth within 7 days. All but three had a vaginal delivery; the instrumental vaginal delivery rate was 16.3%. The results suggest that in postterm women dated with a second-trimester ultrasonic scan, the cervical scores are in general more favorable than previously reported in series not dated with early scans. The postterm group is also much smaller, and the time interval from entry into the postterm period to spontaneous onset of labor is shorter.