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Re: Validation of prediction model for successful vaginal birth after Cesarean delivery based on sonographic assessment of hysterotomy scar. A. Baranov, K. Å. Salvesen and O. Vikhareva. Ultrasound Obstet Gynecol 2018; 51: 189–193.

In this prospective cohort study, Baranov et al. validate a prediction model for successful vaginal birth after Cesarean delivery (VBAC) in a Swedish population. Pregnant women aged 18–35 years who had one previous Cesarean delivery underwent transvaginal ultrasonography of the Cesarean scar in the first and second trimesters. Out of 120 women with complete ultrasound data, 80 women underwent trial of labor after Cesarean delivery (TOLAC), with VBAC in 70 (88%) cases. In contrast to the original study performed by Naji et al.1, the prediction model performed poorly in the present study (area under the receiver–operating characteristics curve 0.44, in comparison to 0.94 in the study of Naji et al.). This striking difference in performance cannot be explained by examining the methodology and results of both studies. The selection of patients in each study and the obstetric management were very different. Naji et al.1 included 320 women, but the prediction model was developed using only those with visible scars (n = 131). In the Swedish study, 80 women were included but only 54 had a visible scar at ultrasonography. However, the model was validated in all 80 women; thus, this study included 26 women without a visible scar. Naji et al.1 excluded only 10 of 131 (8%) women, because they underwent a Cesarean section before TOLAC, and only 74 (61%) women had a successful VBAC. Syntocinon was given in only 12% of patients with TOLAC. Baranov et al. excluded as many as 40 of 117 (34%) women, mainly because they underwent elective Cesarean delivery prior to TOLAC. All participants were counseled by attending obstetricians before making a decision about TOLAC. Therefore, women with low likelihood of successful VBAC may be underrepresented in the study cohort. On the other hand, Syntocinon was given in 53% of patients with TOLAC. It is not surprising, given the much more selective recruitment of patients for TOLAC and the much more liberal use of labor augmentation, that so many (88%) women had a successful VBAC. This management was associated with one rupture and two uterine dehiscences in only 70 women. Because of the very low number of failed TOLAC in the present study (10 cases in total and only seven in the group of women with a visible scar), the numbers are too small to give a reliable calibration analysis. Therefore, the authors focused on discrimination between different outcomes. However, the careful prior assessment and counseling by attending obstetricians seems to have replaced the prediction model in discriminating between failed and successful VBAC in this study, again minimizing the number of events. In 2005, Vergouwe et al.2 concluded that substantial effective sample sizes are required for external validation studies of predictive logistic regression models. For discrimination (c-statistics) they suggested at least 100 events (i.e. 100 failed VBAC) and at least 100 non-events (i.e. 100 successful VBAC). This is in sharp contrast with the 10 actual events in this validation study. The differences in patient selection and obstetric management also highlight the fact that prediction models should be applied only to patient populations selected with similar inclusion and exclusion criteria and clinical management. The findings of the current research indicate that much larger studies with appropriate patient selection are needed to validate and refine prediction models for successful VBAC.