Sonographic assessment of lower uterine segment at term in women with previous cesarean delivery

Objective: To correlate lower uterine segment (LUS) thickness measured by abdominal sonography at term pregnancy with that measured manually using caliper at cesarean delivery and to find out minimum LUS thickness indicative of its integrity in women with previous cesarean. Methods: In 106 women with previous cesarean delivery and 68 with unscarred uterus, abdominal sonographic assessment of LUS was carried out within a week of delivery. Sonographic measurements were correlated with manual measurement of lower flap of LUS using Vernier calipers in 96 of these women (64 with previous cesarean and 32 of unscarred uterus) who had elective cesarean delivery. Results: Sonographically determined LUS was thinner among women with previous cesarean delivery than in those without (4.58 SD 1.05 vs. 4.8 SD 0.8; t = 1.986; p = 0.04). Women with vaginal birth after cesarean had thicker LUS than women with repeat cesarean delivery (4.4 SD 0.97 vs. 4.48 SD 1.0). The findings were not influenced by engaged fetal head status or amount of bladder fullness. Directly measured LUS thickness using Vernier calipers before delivery of the baby confirmed ultrasound measurements, but showed smaller differences between them. There were eight cases with defective uterine scar at cesarean. LUS thickness at term of 3 mm provided 87.5% sensitivity and specificity, and was found to have negative predictive value of 98%. But in two of seven cases the actual LUS was not measurable despite sonographic measurement of 3 mm, and there were two records of scar dehiscence in those with 3 and 4 mm of LUS thickness. Conclusion: LUS thickness of 3 mm measured by abdominal ultrasonography prior to delivery at term in women with previous cesarean is suggestive of stronger LUS but is not a reliable safeguard for trial of labor.