Short-term outcome of the second twin during vaginal delivery is dependent on delivery time interval but not chorionicity.

The decrease in pH in monochorionic (MC) second twin may exceed that of dichorionic (DC) second twin during vaginal twin delivery. The aim of this study was to investigate the effect of chorionicity on the umbilical artery pH of the second twin in relation to increased twin-to-twin delivery time interval (DTI) from 30 weeks of gestation. A hospital based cohort of all twin deliveries after 30 completed weeks of gestation with the first twin delivered vaginally between 2007 and 2012 in four delivery units in Stockholm county was conducted. The changes of umbilical pH in MC twins compared to DC twins in relation to twin-to-twin DTI was studied using linear regression. The predictors for acidosis (pH ≤ 7.10) in the second twin were studied by logistic regression. The cohort included 681 DC and 186 MC twin pairs. There was a correlation between the umbilical pH and the twin-to-twin DTI but independent of the chorionicity, the pH decreased in average by 0.0012 units/min (p < .001). Twin-to twin DTI was a predictor for acidosis of the second twin (p < .01). IMPACT STATEMENT What is already known on this subject? Vaginal twin deliveries are a major obstetrical challenge because of the increased risk for the second twin after the delivery of the first twin, and are suggested to increase among premature second twins. Previous studies report a continuous decline in the pH of the second twin during the second stage of labour, with an increased risk for acidosis (pH < 7.10) and Apgar scores <7 for twin-to-twin delivery time interval (DTI) as short as 15-30 min. Major studies have not discriminated between MC and DC vaginally delivered twins and the question rise if there is a reason for different delivery guidelines for MC diamniotic second twin compared to DC second twin. What do the results of this study add? The results of the present study show a correlation between umbilical pH and twin-to-twin DTI, but independent of chorionicity, from gestational week 30 to term. A main predictor for acidosis (pH ≤ 7.10) is the twin-to-twin DTI. What are the implications of these findings for clinical practice and/or future research? In cases of uneventful MC pregnancies, we find no support to suggest different vaginal delivery guidelines for MC diamniotic twins compared to DC twins. The risk for acidosis in the second twin increased with 1%/min during the second stage. Active management may be recommended after 15-30 min.