Background and Objectives: The aim of this study is to evaluate changes in uterine scar thickness after previous cesarean delivery longitudinally during pregnancy, and to correlate cesarean section (CS) scar myometrial thickness in the first trimester in two participants groups (CS scar with a niche and CS scar without a niche) with the low uterine segment (LUS) myometrial thickness changes between the second and third trimesters. Materials and Methods: In this prospective longitudinal study, pregnant women aged 18−41 years after at least one previous CS were included. Transvaginal sonography (TVS) was used to examine uterine scars after CS at 11−14 weeks. 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. Scar myometrial thickness was measured, and scars were classified subjectively as a scar with a niche (niche group) or without a niche (non-niche group). In the CS scar niche group, RMT (distance from the serosal surface of the uterus to the apex of the niche) was measured and presented as CS scar myometrial thickness in the first trimester. The myometrial thickness at the internal cervical os was measured in the non-niche group. The full LUS and myometrial LUS thickness at 18−20 and 32−35 weeks of gestation were measured in the thinnest part of the scar area using TVS. Friedman’s ANOVA test was used to analyse scar thickness during pregnancy and Mann−Whitney test to compare scar changes between CS scar niche and non-niche women groups. For a pairwise comparison in CS scar thickness measurements in the second and third trimesters, we used Wilcoxon Signed Ranks test. Results: A total of 122 eligible participants were recruited to the study during the first trimester of pregnancy. The scar niche was visible in 40.2% of cases. Uterine scar myometrial thickness decreases during pregnancy from 9.9 (IQR, 5.0−12.9) at the first trimester to 2.1 (IQR, 1.7−2.7) at the third trimester of pregnancy in the study population (p = 0.001). The myometrial CS scar thickness in the first trimester (over the niche) was thinner in the women’s group with CS scar niche compared with the non-niche group (at internal cervical os) (p < 0.001). The median difference between measurements in the CS scar niche group and non-niche group between the second and third trimester was 2.4 (IQR, 0.8−3.4) and 1.1 (IQR, 0.2−2.6) (p = 0.019), respectively. Myometrial LUS thickness as percentage decreases significantly between the second and third trimester in the CS scar niche group compared to the non-niche group (U = 1225; z = −2.438; p = 0.015). Conclusions: CS scar myometrial thickness changes throughout pregnancy and the appearance of the CS scar niche was associated with a more significant decrease in LUS myometrial thickness between the second and third trimesters.
Keywords: cesarean section scar; scar thickness; transvaginal ultrasonography.