Predicting vaginal birth after caesarean section: Validation of the Grobman model in a New Zealand population.

BACKGROUND: The decision regarding mode of birth following a primary caesarean section is important. Women may choose an elective repeat caesarean section or a trial of labour in an attempt to achieve a vaginal birth after caesarean (VBAC). The highest morbidity and mortality is associated with those who have an emergency caesarean section following a trial of labour. Therefore, the ability to accurately predict successful VBAC is important in antenatal counselling. AIMS: To test the validity of the Grobman prediction nomogram in a New Zealand (NZ) population. MATERIALS AND METHODS: A retrospective cohort study was performed of women carrying a singleton, cephalic pregnancy at term and who had one previous lower segment caesarean section in Northland, NZ. The probabilities of successful VBAC were calculated using the variables in the Grobman model and compared with observed VBAC rates using a calibration curve. The predictive ability of the model was assessed using area under the receiver operating characteristic curve (AUC). RESULTS: Of the 421 eligible women, 354 elected to undergo a trial of labour, of whom 69.5% had a successful VBAC. The AUC for the Grobman model was 0.72 (95% CI 0.67-0.78) with predicted and actual outcomes being similar when predicted success was over 50%. The predictive ability of the model appeared more accurate for Maori and Pacifika women compared to the NZ European population. CONCLUSIONS: The Grobman model predicts successful VBAC reasonably well in a NZ population and can be used as an antenatal counselling aid.