Ultrasound examination at 37 weeks' gestation in the prediction of pregnancy outcome: the value of cervical assessment.

OBJECTIVE To examine the potential value of routine measurement of cervical length in singleton low-risk pregnancies at 37 weeks of gestation in the prediction of onset and outcome of labor. METHODS Cervical length was measured by transvaginal sonography at 37 weeks in 1571 singleton low-risk pregnancies. Outcome measures were gestation at spontaneous onset of labor, post-term delivery, duration of labor and mode of delivery. RESULTS The median cervical length at 37 weeks was 30 mm and there was a significant association between cervical length and gestation at delivery, which increased from a mean of 38 weeks for cervical length of 10 mm to 41 weeks for cervical length of 35 mm. The incidence of delivery after 40 weeks and 10 days was 296 (18.8%) and the incidence increased with cervical length at 37 weeks from 0% to 6%, 35% and 68% for respective cervical lengths of < 20, 21-30, 31-40 and 41-50 mm. In the pregnancies with spontaneous onset of labor the incidence of Cesarean section for failure to progress increased from 3.6% to 6.0%, 6.4% and 11.8% for cervical lengths of < 20, 21-30, 31-40 and 41-50 mm, respectively. In the pregnancies requiring induction for post-term the incidence of Cesarean section for failed induction or failure to progress increased from 7.5% to 20.1% to 25.0% for cervical lengths of 21-30, 31-40 and 41-50 mm, respectively. CONCLUSION Measurement of cervical length at 37 weeks can define the likelihood of spontaneous delivery before 40 weeks and 10 days and the risk of Cesarean section in those requiring induction for prolonged pregnancy.