Study design
(1) Development cohort
In this retrospective cohort study, this group consisted of twin pregnant women underwent IOL at ≥ 36 weeks of gestation at SNUH from 2005 to 2018. Twin pregnant women with the following inclusion criteria were included: 1) both viable fetuses; 2) the first baby in the cephalic presentation; 3) no contraindication for IOL (e.g., abnormal placentation, fetal compromise, fetal congenital anomaly, previous uterine surgery, or cesarean delivery); and 4) absence of spontaneous labor, which is defined as regular uterine contractions with cervical change. Women who gave birth through combined delivery (cesarean delivery of the second baby followed by vaginal delivery of the first baby) were excluded.
(2) Validation cohort.
In this validation cohort, the study population consisted of twin pregnant women who underwent IOL at ≥ 36 weeks of gestation at SNUBH from 2005 to 2018. In SNUBH, it is routine practice to perform IOL only in cases with the cephalic presentation of both fetuses, therefore twin pregnancies with cephalic (1st baby) / non-cephalic (2nd baby) presentation are not candidates for IOL.
We collected clinical characteristics including maternal age, maternal height, maternal weight, pregestational body mass index (BMI, divided by weight in kilograms (kg) by the square of their height in meters (m2)), parity, gestational age at IOL, gestational age at delivery, method of conception, cervical examination, chorionicity, presentation, and birth weight of each twin, and summated birth weight of both twins. The variables were compared between pregnant women who had a successful vaginal delivery and those who underwent cesarean delivery after IOL. This retrospective study was approved by the Institutional Review Board of SNUH and SNUBH.