Antenatal and intrapartum prediction of shoulder dystocia.

OBJECTIVES: To (1) develop algorithms to calculate the risk of shoulder dystocia at individual deliveries; (2) evaluate screening for shoulder dystocia. STUDY DESIGN: Retrospective analysis of 40284 consecutive term cephalic singleton pregnancies using a 'train and test' method. Four models were derived using logistic regression and tested (birthweight alone; birthweight and other independent antenatal variables; birthweight and all independent antenatal and intrapartum variables; and all independent variables excluding birthweight). RESULTS: Shoulder dystocia occurred in 240 deliveries (0.6%). Birthweight was the most important risk factor although 98 cases (41%) occurred in babies weighing <4.0kg. Birthweight and maternal height were the only independent antenatal variables; for intrapartum use, only these and instrumental delivery were independent. The antenatal model could calculate an individual's risk; the intrapartum model could also calculate the risk if an instrumental delivery were undertaken. Both showed 0.7% women to have a risk of shoulder dystocia of >10%. Although the antenatal model had high predictability (area under curve 0.89), it was no better than birthweight alone and had a sensitivity of 52.4%. Where birthweight was excluded, prediction of shoulder dystocia was poor. CONCLUSION: Antepartum and labour calculation of the risk of shoulder dystocia is possible. Whilst greatly hindered by the inaccuracy of estimating weight, it allows due weight to be given to factors which may already be influencing clinical practice. However, shoulder dystocia cannot be predicted with sufficient accuracy to allow universal screening