Lower uterine segment thickness to predict uterine rupture: a secondary analysis of PRISMA cluster randomized trial

Third-trimester lower uterine segment thickness (LUST) is associated with uterine rupture during trial of labor after cesarean (TOLAC) but threshold values vary according to the approach used (lower values with vaginal ultrasound, higher values with abdominal ultrasound).

To estimate the optimal LUST cut-off value combining vaginal and abdominal ultrasound to predict uterine rupture during TOLAC.

We performed a secondary analysis of PRISMA cluster randomized trial including women with a single previous cesarean who underwent ultrasound LUST measurement at 34-38 weeks using the thinnest measurement obtained by combining transvaginal and transabdominal measurements. Participants in the intervention group were informed about the risk of uterine rupture according to LUST reported in 3 categories (≥2.5 mm: low risk-TOLAC is safe; 2.0-2.4 mm: intermediate-risk-TOLAC is safe under specific conditions (e.g., Grobman estimate of vaginal delivery of at or above 70% and/or having a history of vaginal delivery; estimated fetal weight below 4000 grams; interdelivery interval≥18 months); <2.0 mm: high-risk for uterine rupture). Delivery outcomes including uterine rupture were compared using nonparametric analyses and receiver operating characteristics (ROC) curves.

Among 3460 participants, 2809 (81%); 385 (11%); and 266 (8%) were identified at low-; intermediate-; and high-risk for uterine rupture, respectively. As expected, low-risk participants were more likely to undergo TOLAC (49% vs 46% vs 13%; P=.001) and more likely to undergo labor induction (16% vs 12% vs 3%, respectively; P=.001) than intermediate-risk and high-risk participants. Four (0.3%) cases of uterine rupture during TOLAC occurred among 1382 low-risk participants but none among the intermediate-risk (0/178) and high-risk (0/35) participants (P=.73). Among low-risk participants, uterine rupture was associated with LUST combining vaginal and abdominal ultrasound (area under the ROC curve: 0.93; 95% confidence interval: 0.86%-0.99%; P=.001) with all cases occuring among women with LUST between 2.5 and 3.0 mm (4/371 or 1.1%) compared to none (0/1011) among those with LUST≥3.0 mm (P=.01).

Third-trimester LUST measurement influences the rates of TOLAC and uterine rupture. TOLAC appears to be associated with a low risk of uterine rupture with a LUST between 2.0 and 2.4 mm under specific conditions. However, these conditions should perhaps be extended to patients with a LUST of less than 3.0 mm, as we found an increase in uterine ruptures in this subgroup (between 2.5 and 3.0 mm) who had no special conditions to comply with. LUST≥3.0 mm combining vaginal and abdominal ultrasound was associated with a very low risk for uterine rupture. A large-scale study using these new parameters would have the potential to demonstrate a reduction in uterine ruptures during TOLAC.