Transvaginal Sonographic Evaluation of Cesarean Section Scar Niche in Pregnancy: A Prospective Longitudinal Study

Background and Objectives: To investigate the prevalence of a Cesarean section (CS) scar niche during pregnancy, assessed by transvaginal ultrasound imaging, and to relate scar measurements, demographic and obstetric variables to the niche evolution and final pregnancy outcome. Materials and Methods: In this prospective observational study, we used transvaginal sonography to examine the uterine scars of 122 women at 11+0–13+6, 18+0–20+6 and 32+0–35+6 weeks of gestation. A scar was defined as visible on pregnant status when the area of hypoechogenic myometrial discontinuity of the lower uterine segment was identified. The CS scar niche (“defect”) was defined as an indentation at the site of the CS scar with a depth of at least 2 mm in the sagittal plane. We measured the hypoechogenic part of the CS niche in two dimensions, as myometrial thickness adjacent to the niche and the residual myometrial thickness (RMT). In the second and third trimesters of pregnancy, the full lower uterine segment (LUS) thickness and the myometrial layer thickness were measured at the thinnest part of the scar area. Two independent examiners measured CS scars in a non-selected subset of patients (n = 24). Descriptive analysis was used to assess scar visibility, and the intraclass correlation coefficient (ICC) was calculated to show the strength of absolute agreement between two examiners for scar measurements. Factors associated with the CS scar niche, including maternal age, BMI, smoking status, previous vaginal delivery, obstetrics complications and a history of previous uterine curettage, were investigated. Clinical information about pregnancy outcomes and complications was obtained from the hospital’s electronic medical database. Results: The scar was visible in 77.9% of the women. Among those with a visible CS scar, the incidence of a CS scar niche was 51.6%. The intra- and interobserver agreement for CS scar niche measurements was excellent (ICC 0.98 and 0.89, respectively). Comparing subgroups of women in terms of CS scar niche (n = 49) and non-niche (n = 73), there was no statistically significant correlation between maternal age (p = 0.486), BMI (p = 0.529), gestational diabetes (p = 1.000), smoking status (p = 0.662), previous vaginal delivery after CS (p = 1.000) and niche development. Uterine scar niches were seen in 56.3% (18/48) of the women who had undergone uterine curettage, compared with 34.4% (31/74) without uterine curettage (p = 0.045). We observed an absence of correlation between the uterine scar niche at the first trimester of pregnancy and mode of delivery (p = 0.337). Two cases (4.7%) of uterine scar dehiscence were confirmed following a trial of vaginal delivery. Conclusions: Based on ultrasonography examination, the CS scar niche remained visible in half of the cases with a visible CS scar at the first trimester of pregnancy and could be reproducibly measured by a transvaginal scan. Previous uterine curettage was associated with an increased risk for uterine niche formation in a subsequent pregnancy. Uterine scar dehiscence might be potentially related to the CS scar niche.