Background and aims Adverse outcomes have been associated with a prolonged inter-twin delivery time. The aim of the present study was to analyse the effects of inter-twin delivery time on umbilical artery pH and Apgar scores with a view to establish a time at which there is an increased risk of fetal acidosis.

Method A retrospective study of twin births delivered at Jessops Hospital over 7 years was undertaken. Live twin births, delivered at 34 weeks or greater, with the first twin delivered vaginally were included in the study. Gestational age, birth weight, 1 and 5 min Apgar scores and umbilical artery pH were identified. Spearmans rank correlation coefficients, Mann-Whitney U tests and ROC curves were performed for statistical analysis.

Results 314 cases were reviewed. Inter-twin delivery time correlated negatively with umbilical artery pH (r=−0.204, p=0.023) and with the 5 min Apgar score of twin two (r=−0.125, p=0.034). Umbilical arterial pH was significantly lower where inter-twin delivery time exceeded 30 min. When inter-twin delivery time was 30 min or more, 70% of second twins had a pH of 7.20 or less, 36.6% when second twins were delivered in less than 30 min. The ROC area under the curve was 0.668 when using a pH of 7.2 as a cut-off for fetal acidosis.

Conclusion The present study shows that a prolonged inter-twin delivery time is associated with deterioration of the umbilical arterial pH in the second twin, supporting the theory of a reduced placental perfusion after the delivery of the first twin.