Validation of a Prediction Model for Predicting the Probability of Morbidity Related to a Trial of Labour in Quebec

Abstract

Between 2000 and 2006, births by cesarean section (CS) increased from 21.2% to 26.3% in Canada and reached greater than 30% in the United States. A prior CS is the primary indication for greater than 30% of all CS procedures in Canada. The number of CSs could be decreased if more women who could undergo a trial of labor after CS (TOLAC) chose this option. The decision to attempt a TOLAC or to have an elective repeat CS (ERCS) is often made by considering the fear of a repeat failed labor or the odds of developing maternal and neonatal complications with either delivery mode. Grobman’s model, which allows an estimation of the probability of vaginal delivery for women with 1 prior CS undergoing a TOLAC, was developed from 9616 American women experiencing a TOLAC and validated using an independent cohort. Maternal and neonatal morbidity decreased as the predicted chance of vaginal birth after cesarean (VBAC) increased. Morbidity was similar between TOLAC and ERCS when the probability of successful VBAC was 70% or greater. Because the Grobman model has not been validated in a Canadian population and because American and Canadian populations and clinical practices may differ, validation is needed before the prediction model can be used in Canada. The present study was performed to determine whether Grobman’s model for predicting TOLAC success in an American population can predict the probability of morbidity associated with TOLAC in a Canadian population. A perinatal database with 185,437 deliveries from 32 obstetric centers in Quebec was used, and women with 1 prior CS who were eligible for a TOLAC were selected. The probability of VBAC was calculated from the Grobman model, using maternal age, body mass index, history of vaginal delivery, timing of vaginal delivery in relation to the CS, and indication for the prior CS. Major and minor maternal morbidities and newborn morbidities were determined. Women were then categorized by deciles of their predicted probability of VBAC, and the frequencies of maternal and neonatal morbidities between those who underwent TOLAC and those who had ERCS were compared using either the Fisher exact or χ2 analysis. Statistical significance was defined as P < 0.05. Of 8508 women with 1 prior CS, 3113 had a TOLAC (36.6%) and 5395 underwent an ERCS (63.4%). Minor and major maternal morbidity occurred in 1.9% and 1.1%, respectively; the frequency of neonatal morbidity was 5.1%. Maternal and neonatal morbidities became less frequent as the predicted chance of VBAC increased among women who underwent TOLAC (P < 0.05). No comparable trend was apparent among women who had ERCS. When the predicted chance of VBAC was less than 60%, women undergoing a TOLAC were more likely to have morbidity than women who underwent an ERCS (P < 0.05). When the probability of VBAC success reached 60%, this difference was not present. Neonatal morbidities showed a similar pattern, but they became equivalent between TOLAC and ERCS only when the probability of VBAC was 70% or greater. All women were grouped according to predicted probabilities of VBAC success of either 60% or 70%. When the predicted chance of VBAC was less than 60%, women undergoing a TOLAC were more likely to have maternal morbidity (RR 2.3; 95% confidence interval [CI], 1.4–4.0) than those who underwent an ERCS. However, when the predicted chance of VBAC was 60% or greater, maternal morbidity did not differ between the groups (RR, 0.8; 95% CI, 0.6–1.1). Neonatal morbidity was similar between groups when the probability of VBAC reached 70% (RR, 1.2; 95% CI, 0.9–1.5). This prediction model for VBAC success as validated in the United States is also predictive for Canadian women in Quebec. Maternal and neonatal morbidity became less frequent as the predicted chance of VBAC increased among women who underwent TOLAC. The findings of this study confirm that individual assessment of the probability of VBAC success allows health care providers to refine and improve counseling to inform women with a prior CS about their chance of successful VBAC and their level of risk, to enable them to make an informed choice regarding their mode of delivery.