Study Highlights Lower Uterine Segment Thickness as Key Predictor for Uterine Rupture Risk During TOLAC

The research published in the American Journal of Obstetrics & Gynecology MFM found that a LUST measurement between 2.0 and 2.4 mm in the third trimester was associated with a low risk of uterine rupture in women attempting TOLAC under specific conditions. However, the study also revealed that women with LUST measurements between 2.5 and 3.0 mm, who did not meet these special conditions had a higher incidence of uterine rupture. 

These findings suggest that the current threshold for safe TOLAC might need to be adjusted, potentially extending the definition to include women with LUST measurements under 3.0 mm.

The study also found that LUST measurements greater than or equal to 3.0 mm, when combined with both vaginal and abdominal ultrasound, were associated with an exceptionally low risk of uterine rupture.

Third-trimester lower uterine segment thickness has been linked to the risk of uterine rupture during trial of labor after cesarean, with threshold values differing depending on the type of ultrasound used—lower values with vaginal ultrasound and higher values with abdominal ultrasound. Given this variability, Emmanuel Bujold, Department of Obstetrics and Gynecology, Faculty of Medicine, Université Laval, Québec, Canada, and colleagues sought to determine the optimal LUST cut-off value by combining both vaginal and abdominal ultrasound measurements to more accurately predict uterine rupture during TOLAC.

For this purpose, the researchers conducted a secondary analysis of the PRISMA cluster randomized trial, which included women with a single previous cesarean who underwent LUST measurement at 34-38 weeks. The thinnest measurement was obtained by combining transvaginal and transabdominal ultrasounds. Participants were categorized into three LUST risk groups: ≥2.5 mm (low risk, TOLAC safe), 2.0-2.4 mm (intermediate risk, TOLAC safe under specific conditions), and <2.0 mm (high risk for uterine rupture).

Delivery outcomes, including uterine rupture, were analyzed using non-parametric methods and receiver operating characteristics (ROC) curves.

The key findings of the study were as follows:

• Among 3,460 participants, 81% were classified as low-risk, 11% as intermediate-risk, and 8% as high-risk for uterine rupture.
• Low-risk participants were more likely to attempt TOLAC (49%) and undergo labor induction (16%) compared to intermediate-risk (46% and 12%) and high-risk participants (13% and 3%).
• Four cases of uterine rupture occurred during TOLAC among 1,382 low-risk participants, but there were no uterine ruptures in intermediate-risk (0/178) or high-risk (0/35) groups.
• In low-risk participants, uterine rupture was strongly associated with LUST measurements (vaginal + abdominal ultrasound), with an area under the ROC curve of 0.93.
• All uterine rupture cases occurred in women with a LUST between 2.5 and 3.0 mm (4/371, 1.1%), while no ruptures occurred in those with LUST ≥3.0 mm (0/1,011).

"We emphasize the critical importance of using a combined transabdominal and transvaginal approach for LUST measurement, supported by both extensive literature and the findings of this study," the researchers wrote.

"A large-scale study utilizing these new parameters could potentially show a reduction in uterine ruptures during TOLAC," they concluded.

Reference:

Bujold, E., Dubé, E., Girard, M., & Chaillet, N. (2024). Lower uterine segment thickness to predict uterine rupture: A secondary analysis of PRISMA cluster randomized trial. American Journal of Obstetrics & Gynecology MFM, 101543. https://doi.org/10.1016/j.ajogmf.2024.101543