Sonographic evaluation of the lower uterine segment thickness in women with a single previous Cesarean section

The aim of this paper was to evaluate the lower uterine segment (LUS) thickness through transvaginal sonography in late preterm and full term pregnancies with a single previous Cesarean section, to correlate the obtained LUS measurements with intraoperative observations, and to identify a predictive cut-off value in order to select the best candidates for a vaginal birth after Cesarean delivery (VBAC).

Two hundred and fourteen women with a single previous Cesarean section who had an ultrasound measurement of the LUS thickness (stratified in S1, S2 and S3) in pregnancy were enrolled. The outcome of interest was the visual finding of a thin uterine scar at the time of the iterative Cesarean section. Linear regression was used to correlate the LUS thickness with gestational age (GA). A ROC curve has been used to determine the detection rate (DR) and the risk of each actual value of LUS thickness versus a thin uterine scar (outcome of interest).

The LUS thickness was correlated with the gestational age (R2=0.034, P-value =0.005). The DR as estimated by ROC curves to detect a translucent lower uterine segment (S3) was 94.1% at a false positive rate (FPR) of 20%. The correspondent cut-off value was 1.8 mm. Finally a likelihood ratio (LR) of observing S3 was estimated. At the quoted cut-off of 1.8 mm the LR was 3. As demonstrated, for a segment of 1 mm the LR was instead about 13.

The obtained values lead us to the conclusion that a thickness less than 1.8 mm can be reasonably considered a valid cut-off value to identify patients with a higher risk of thin uterine scar.