The prospective sonographic assessment of the lower uterine segment after cesarean section and its clinical utility

Purpose This study aims to investigate the sonographic features of the lower uterine segment (LUS) and their association with successful vaginal birth after a cesarean section (VBAC). Methods This is a prospective observational cohort study of women who underwent a first cesarean section (CS). Inclusion criteria were age over 18 years and open family planning. Women with a history of any additional uterine surgery, as well as a vertical or inverted T uterotomy during the CS were excluded. A pregestational transvaginal sonography, followed by serial transabdominal ultrasound examinations at the first, second, and third trimesters upon starting a new pregnancy were performed. Each intra-gestational examination involved measuring the LUS on a sagittal plane over a length of 3 cm starting from the most inferior identifiable part of the myometrium behind a full bladder. Logistic regression was performed to test the association between measures of the LUS and successful vaginal birth after CS. Results 96 women with a follow-up pregnancy within 2 years of the initial CS were included in the analysis. The pregestational RMT ratio was 62% and 38 (39%) women had a niche. The median thickness of the lower uterine segment was 8.34 mm (5.57-9.77), 4.75 mm (4.02-5.95), and 2.55 mm (2.01-3.55) at the first, second and third trimester, respectively. 70 women attempted VBAC and the risk of unplanned repeat CS was 37.1%. One millimeter increase in LUS thickness in the first trimester increased the odds of VBAC by 50-120% depending on the used measure (p < 0.05). This association weakened with increasing gestational age and the p values increased above 0.05. Conclusion There is a good chance of successful VBAC for women attempting it. The sonographic assessment of the lower uterine segment during pregnancy could be helpful in counseling these women, albeit it seems that performing ultrasound during the first trimester is more informative than second and third trimesters.