We have investigated the factors predicting umbilical arterial pH (UA pH) and standard base excess (UA BE) in 337 consecutive elective Caesarean sections performed under spinal anaesthesia. Multiple linear regression analysis was performed with UA pH and UA BE as the dependent factors. We found that the significant factors predicting UA pH were: use of ephedrine, uterine incision-to-delivery time, maximum decrease in systolic arterial pressure and the interaction between ephedrine use and duration of hypotension (adjusted R2 = 0.39, F15,321 = 15.4, p < 0.0001). The significant factors predicting UA BE were: use of ephedrine and the interaction between ephedrine use and duration of hypotension (adjusted R2 = 0.52, F15,321 = 25.0, p < 0.0001). We conclude that, in order to minimise the risk of fetal acidosis, ephedrine should not be used before delivery, uterine incision-to-delivery time should be as short as possible, and alpha-agonists such as metaraminol or phenylephrine should be used to minimise both the magnitude and duration of hypotension.