title: Validation of a prediction model for predicting the probability of morbidity related to a trial of labour in Quebec.
abstract: BACKGROUND
  Pregnant women with a history of previous Caesarean section face the decision either to undergo an elective repeat Caesarean section (ERCS) or to attempt a trial of labour with the goal of achieving a vaginal birth after Caesarean (VBAC). Both choices are associated with their own risks of maternal and neonatal morbidity. We aimed to determine the external validity of a prediction model for the success of trial of labour after Caesarean section (TOLAC) that could help these women in their decision-making.
  
  
  METHODS
  We used a perinatal database including 185,437 deliveries from 32 obstetrical centres in Quebec between 2007 and 2011 and selected women with one previous Caesarean section who were eligible for a TOLAC. We compared the frequency of maternal and neonatal morbidity between women who underwent TOLAC and those who underwent an ERCS according to the probability of success of TOLAC calculated from a published model of prediction.
  
  
  RESULTS
  Of 8508 eligible women, including 3113 who underwent TOLAC, both maternal and neonatal morbidities became less frequent as the predicted chance of VBAC increased (P < 0.05). Women undergoing a TOLAC were more likely to have maternal morbidity than those who underwent an ERCS when the predicted probability of VBAC was less than 60% (relative risk [RR] 2.3; 95% CI 1.4 to 4.0); conversely, maternal morbidity was not different between the two groups when the predicted probability of VBAC was at least 60% (RR 0.8; 95% CI 0.6 to 1.1). Neonatal morbidity was similar between groups when the probability of VBAC success was 70% or greater (RR 1.2; 95% CI 0.9 to 1.5).
  
  
  CONCLUSIONS
  The use of a prediction model for TOLAC success could be useful in the prediction of TOLAC success and perinatal morbidity in a Canadian population. Neither maternal nor neonatal morbidity are increased with a TOLAC when the probability of VBAC success is at least 70%.
authors.affiliation: Research Centre of Sainte-Justine Hospital, University of Montreal, Montreal QC.
authors.forename: Nils
authors.initials: N
authors.lastname: Chaillet
authors.affiliation: Department of Obstetric and Gynaecology, University of Laval, Quebec QC.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Bujold
authors.affiliation: Research Centre of Sainte-Justine Hospital, University of Montreal, Montreal QC.
authors.forename: Eric
authors.initials: E
authors.lastname: Dubé
authors.affiliation: Department of Obstetric and Gynaecology, Northwestern University, Chicago IL.
authors.forename: William A
authors.initials: WA
authors.lastname: Grobman
mesh_terms: D017604:Cesarean Section, Repeat / Q000706:statistics & numerical data; D017558:Elective Surgical Procedures / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D015931:Intensive Care, Neonatal / Q000706:statistics & numerical data; D009017:Morbidity; D011247:Pregnancy; D011336:Probability; D011792:Quebec / Q000453:epidemiology; D014219:Trial of Labor; D016064:Vaginal Birth after Cesarean / Q000009:adverse effects* / Q000706:statistics & numerical data