In nulliparous women with uncomplicated pregnancies undergoing an induction of labor at term, a simplified Bishop score with three components: dilation, station and effacement predicted vaginal delivery similarly to the original Bishop score. The simplified Bishop score also was comparable to the original Bishop score in predicting successful vaginal delivery in women with an indicated induction both at term and preterm between 32 – 36 6/7 weeks of gestation. Even in women who presented in spontaneous labor at term and preterm, the simplified Bishop score was similar to the original Bishop score, suggesting that the simplified score is equivalent to the original score in the setting that it was developed.
Other attempts at modifying or evaluating the Bishop score have used different outcomes such as length of labor or achieving active labor, and many included multiparous women who are known to have more successful inductions.3–5, 7
We chose vaginal delivery as the primary outcome, because this is what clinicians and patients define as success. Our study also has the advantage of having a large number of nulliparous women. Thus, we were able to use modern statistical methods to find which components of the Bishop score were independently associated with vaginal delivery in order to create a simplified score.
There is a possibility that women who had all five components of the Bishop score recorded are different in baseline characteristics from women who were missing some of the components. However most of the women (72.2%) had dilation, station, and effacement present, and many clinicians informally already use a simplified Bishop score. It is more likely that the recording of some versus five components of the Bishop score was based on clinician preference rather than something inherently different about a woman undergoing an induction. Given the large numbers we were able to test the simplified score in other populations of women, including indicated induction and spontaneous labor both term and preterm, and the simplified Bishop score performed similarly to the original Bishop score in predicting vaginal delivery in all of these settings which suggests that missing cervical components were likely not an issue.
Our findings are similar to a prospective study of 134 women undergoing an induction of labor at term, where only the cervical components of dilation and effacement were associated with vaginal delivery within 24 hours.15
Using an “abbreviated” Bishop score including dilation and effacement only > 3, the predictive characteristics of vaginal delivery (excluding 23 women who had an emergency cesarean delivery for maternal or fetal indications) were PPV 85.5%, NPV 65.7%, and LR + 2.61, which were similar to our simplified Bishop > 5. An older, smaller study of 40 nulliparous and 69 multiparous women also found that only dilation was associated with the length of latent phase of labor after labor induction.16
Our study found both effacement and station to be significant in addition to dilation likely because we had a large number of women and thus more power. While the addition of position or consistency may be significantly associated with successful vaginal delivery in a different population of women, the purpose of our model was to simplify the score, so we chose only the components that were both highly significant in the regression and contributed the most to vaginal delivery as determined by the regression coefficients. Of note, simplifying the score even further by using only the two components with the highest regression coefficients, dilation and station, resulted in a worse correct classification rate compared to the simplified Bishop score using all three components of dilation, station and effacement (data not shown). Our findings are also supported by a secondary analysis of four randomized controlled trials with a total of 781 women comparing different induction methods for indicated induction after 37 weeks’ gestation, and the cervical components dilation, effacement and station were independently associated with vaginal delivery within 24 hours after adjusting for maternal and obstetrical characteristics, although only position and station were associated with spontaneous vaginal delivery.17
Other studies have created variations of the Bishop score. In a prospective study of 1189 women undergoing induction mostly for indicated indications, Lange et al. used linear regression to create a new score with the cervical components of dilation and station from the original Bishop score and length measured as centimeters as opposed to percentage, with dilation multiplied by two.7
The indications for induction (PROM, amniotomy and medically induced) and definitions of failure (delivery within 24 hours or labor established within 8 hours for the medically induced group) were different from our study as well as a lower overall rate of failure of around 15% compared to 25% in our study. Nonetheless, Lange’s score was found to perform similarly to the original Bishop score in that population of women. Dhall et al. also created a new score in 200 women undergoing indicated induction with a slightly lower vaginal delivery rate (71.5%) than our study.8
Dilation, effacement and consistency were rescored and weighted, and parity was also included. The Dhall score had higher prediction of success rate at both ends of the score, but the study was limited because no women had a Bishop score > 8. In addition, using a reasonably accurate prediction of Dhall score ≥ 7 which corresponded to a Bishop score cut-off point of 4, the Dhall score only performed significantly better in multiparous but not nulliparous women.
In summary, reassessing the original Bishop score using modern statistical methods resulted in a simplified score with only three components: dilation, station and effacement yielding an equivalently high predictive ability. The simplified Bishop score performed similarly to the original Bishop score in predicting vaginal delivery in indicated inductions term and preterm, as well as in spontaneous labor at term and preterm. Given that our study is a large, nationally representative cohort reflecting current clinical practice, our findings are generalizable. As cervical position and consistency do not add to the overall ability to predict vaginal delivery, we believe that the original Bishop score can be replaced with a simplified score using dilation, station and effacement only.