First and Third Trimester Uterine Scar Thickness in Women With Previous Caesarean: A Prospective Comparative Study

Lower uterine segment (LUS) thickness in the third trimester of gestation is associated with the risk of uterine scar defect at delivery. It was suggested that first trimester residual myometrial thickness (RMT) could also predict uterine scar defect at delivery.

This study sought to correlate the RMT measured at the site of uterine scar in the first trimester with the LUS thickness measured in the third trimester.

This was a prospective cohort study of women with a singleton pregnancy and a single prior low-transverse CS. All participants underwent an evaluation of uterine scar by using transvaginal ultrasound at 11 to 13 weeks, including the presence of a scar defect and measurement of RMT; and a second evaluation at 35 to 38 weeks, combining both transvaginal and transabdominal ultrasound, for the measurement of LUS thickness. Spearman's correlation test was used to compare first and third trimester measurements.

A total of 166 eligible participants were recruited at mean GA of 12.7 ± 0.5 weeks. We observed an absence of correlation between first trimester RMT and third trimester LUS thickness (correlation coefficient 0.10; P = 0.20). First trimester RMTs below 2.0 mm and below 2.85 mm are poor predictors of third trimester LUS thickness below 2.0 mm (sensitivity, 8% and 23%; specificity, 98% and 87%; positive predictive value, 25% and 14%, respectively).

There is a poor correlation between first trimester RMT and third trimester LUS thickness in women with a previous CS. First trimester RMT should not be used to inform women on their risk of uterine rupture or to guide clinical management.