Cervical length measured by transvaginal ultrasound is a better predictor of vaginal delivery within 60 hours after labor induction than the traditional Bishop score. Researchers in Spain compared the two methods in deciding when to use prostaglandin and oxytocin(Drug information on oxytocin) to induce labor.
The Bishop score gives point values to the position of the baby and several conditions of the cervix determined with a digital exam. Previous studies have shown women find a transvaginal ultrasound exam more comfortable than the digital exam, and several recent papers suggest ultrasound is also more accurate. It is less subjective, and it measures the cervix in a passive state.
Dr. Ana M. Gómez Laencina and colleagues at the gynecology and obstetrics service of the Virgin of Light Hospital in Cuenca, Spain, published their findings in July issue of the Acta obstetricia et gynecologica Scandinavica. They studied 177 women with a single pregnancy between 36 and 42 weeks of gestation with a live fetus in cephalic presentation. Prior to induction, a Bishop score was assigned each woman and cervical length was measured by transvaginal ultrasound. Topical prostaglandin and oxytocin were used in the inductions. The researchers defined success as a vaginal delivery within 60 hours.
Their multiple regression analysis showed the Bishop score, cervical length, and parity all contributed independently to the prediction of a successful delivery. Only one component of the Bishop score — station of the baby — contributed to its success.
They found the best cutoff points for predicting success, using receiver operating characteristic curves, were 24 mm for cervical length and 4 for the Bishop score. Cervical length was a better predictor, with a sensitivity of 66% and a specificity of 77% versus the Bishop score's sensitivity of 77% and specificity of 56%. They used Kaplan-Meier survival curves to show that cervical length measured by transvaginal ultrasound predicted success the best.
Another study from Spain published in the January 2007 Journal of Perinatology summarized findings concerning this issue with a decision tree. Dr. Bebita Bueno and colleagues in the ob/gyn department at the Santa Cristina University Hospital in Madrid defined successful induction as vaginal delivery within 24 hours. They found cervical measurement with transvaginal ultrasound to be most effective in predicting success when the patient was a first-time mother.
