Risk factors for cesarean section of the second twin after vaginal delivery of the first twin

Purpose To document risk factors for combined delivery, defined as unplanned cesarean section for the second twin after vaginal delivery of the first twin, particularly focusing on delivery interval between twins. Methods A multi-center cross-sectional study among a cohort of 5411 women with twin pregnancy experiencing vaginal delivery of the first twin in 2007-2016 at 191 tertiary referral hospitals in Japan was conducted. Primary outcome was the occurrence of combined delivery, and data were collected through the Japan Society of Obstetrics and Gynecology perinatal database. Risk factors for combined delivery were investigated using Poisson regression analysis. Results Combined delivery occurred in 235 women (5.1%) and was significantly associated with delivery interval (P < 0.001). Multivariate analysis showed women with >= 25 kg/m(2) pre-pregnancy body mass index (BMI) and with birthweight < 1500 g of the second twin had significantly higher risk for combined delivery than women with 18.5-25 kg/m(2) pre-pregnancy BMI and with birthweight >= 2500 g of the second twin (adjusted risk ratio (aRR) 1.72, 95% confidence interval (CI) 1.15-2.57, and aRR 2.06, 95% CI 1.14-3.72, respectively). Breech and transverse presentation of the second twin were also risk factors for combined delivery compared with cephalic presentation (aRR 3.60, 95% CI 2.67-4.85, and aRR 9.94, 95% CI 6.50-15.0, respectively). Although association of combined delivery with pre-pregnancy BMI was attenuated after adjustment by delivery interval, association with birthweight of the second twin was strengthened. Conclusion Delivery interval was significantly associated with combined delivery and mediated the association between combined delivery and some other risk factors.