Second-trimester sonographic comparison of the lower uterine segment in pregnant women with and without a previous cesarean delivery

Objective. To compare measurements of the lower uterine segment during a second-trimester sonographic examination in women with and without a previous cesarean delivery. Methods. Women undergoing second-trimester sonographic examination, 24 with a history of cesarean delivery and 30 control subjects with no history of cesarean delivery, were recruited for transvaginal sonographic evaluation of the lower uterine segment with a high-frequency probe. The uterine niche or previous cesarean scar site was defined as a small triangular anechoic defect in the anterior wall of the uterus. The uterine wall thickness was measured successively at the level where the bladder dome meets the lower uterine segment. Measurements were obtained with cursors at the interface of the urine-bladder and the amniotic fluid-decidua. The study was approved by the Institutional Review Board, and P < .05 was considered significant. Results. The uterine niche was identified in 14 (58%) of 24 women with a previous cesarean delivery. The lower uterine segment was significantly thinner in women with a previous cesarean delivery compared with control subjects (mean ± SD, 4.7 ± 1.1 versus 6.6 ± 2.0 mm; P < .001). In the previous cesarean group, the mean lower uterine segment thickness was similar in the 5 women with 2 cesarean deliveries when compared with those with 1 cesarean delivery (4.6 ± 1.0 versus 4.7 ± 1.4 mm; P = .91). In a linear regression model, the only variable retaining significance in the prediction of uterine wall thickness was previous cesarean delivery (P = .002). Maternal age, parity, number of previous cesarean deliveries, and gestational age did not attain significance in the model. Conclusions. The lower uterine segment during a second-trimester sonographic examination is significantly thinner in women with a previous cesarean delivery. Identification of the scar niche is possible in most of these women.