OC08.06: Lower uterine segment thickness to predict uterine rupture: a secondary analysis of the PRISMA randomised trial

To estimate the role of lower uterine segment thickness (LUST) to predict uterine rupture during trial of labour after Caesarean (TOLAC). We performed a secondary analysis of PRISMA randomised trial including women who underwent ultrasound LUST measurement at 34-38 weeks. According to protocol, women were informed about their risk of uterine rupture using LUST (≥2.5 mm: low risk; 2.0-2.4 mm: moderate risk; <2.0 mm: high-risk) and special measures (Caesarean, avoiding dystocia) were used accordingly. The diagnosis of uterine rupture was verified by 3 independent MFM blinded to the LUST. ROC curves with area under the curve (AUC) were used to compare the sensitivity and specificity of LUST to predict uterine rupture in the low-risk group according to ultrasound approach (abdominal, transvaginal, both). Among 3460 eligible participants, 1595 (46%) underwent TOLAC. TOLAC rate were 49% (1382/2809), 46% (178/385) and 13% (35/266) in the low-risk, moderate- and high-risk groups, respectively (p < 0.001). Four (0.3%) uterine ruptures occurred among 1382 TOLAC in the low-risk group but none in the moderate- (0/178) and high-risk (0/35) groups. LUST was associated with uterine rupture using transabdominal (AUC: 0.83; 95%CI: 0.72-0.93); transvaginal (AUC: 0.86; 95%CI: 0.72-0.99), and both approaches (AUC: 0.90; 95%CI: 0.81-0.98; p < 0.001) (all p < 0.001); with different threshold to reach sensitivity of 100% (3.5 mm, 3.5 mm, and 2.9 mm, for specificities of 72%, 62% and 73%, respectively). All cases occurred among women with LUST <3.0mm using the combined approach (4/387 or 1.0%) compared to none among 995 TOLAC with LUST ≥3.0mm (p = 0.01). Third trimester LUST is highly associated with the risk of uterine rupture, and the best approach is a combination of vaginal and abdominal ultrasound. A LUST below 3.0mm should be considered a risk factor for uterine rupture during TOLAC. No uterine rupture was observed among high-risk women who attempted TOLAC after being advised of their risk based on LUST measurement.