Evaluation of the usefulness of ultrasound measurement of the lower uterine segment before delivery of women with a prior cesarean delivery: a randomized trial.

Background: The main reason to avoid trial of labor after cesarean delivery is the possibility of uterine rupture. Identifying women at risk is thus an important aim, for it would enable women at low risk to proceed with a secure planned vaginal birth.

Objective: To evaluate the impact of proposing mode of delivery based on the ultrasound measurement of the lower uterine segment thickness on a composite outcome of maternal-fetal mortality and morbidity, compared with usual management, among pregnant women with a previous cesarean delivery.

Study Design: This multicenter, randomized, controlled, parallel-group, unmasked trial was conducted at 8 referral university hospitals with a neonatal intensive care unit and enrolled 2948 women at 36 weeks 0 days to 38 weeks 6 days of gestation with 1 previous low transverse cesarean delivery and no contraindication to trial of labor. Women in the study group had their lower uterine segment thickness measured by ultrasound. Those with measurements >3.5 mm, were encouraged to choose a planned vaginal delivery, and those with measurements ≤3.5 mm, were encouraged to choose a planned repeat cesarean delivery. This measurement was not taken in the control group; their mode of delivery was decided according to standard management. The primary outcome was a composite criterion comprising maternal mortality, uterine rupture, uterine dehiscence, hysterectomy, thromboembolic disease, transfusion, endometritis, perinatal death, or neonatal encephalopathy. Prespecified secondary outcomes were repeat cesarean deliveries, elective or after trial of labor.

Results: The study group included 1472 women, and the control group included 1476 women. These groups were similar at baseline. The primary outcome occurred in 3.4% of the study group and 4.3% of the control group (relative risk, 0.78; 95% confidence interval, 0.54-1.13: risk difference, -1.0%; 95% confidence interval, -2.4 to 0.5). The uterine rupture rate in the study group was 0.4% and in the control group 0.9% (relative risk, 0.43; 95% confidence interval, 0.15-1.19). The planned cesarean delivery rate was 16.4% in the study group and 13.7% in the control group (relative risk, 1.21; 95% confidence interval, 1.00-1.47), whereas the rates of cesarean delivery during labor were 25.1% and 25.0% (relative risk, 1.01; 95% confidence interval, 0.89-1.14) in the study and control groups, respectively.

Conclusion: Ultrasound measurements of lower uterine segment thickness did not result in a statistically significant lower frequency of maternal and perinatal adverse outcomes than standard management. However, because this study was underpowered, further research should be encouraged.