Delivery of the Nonpresenting Twin First: Rates and Associated Factors.

Objective: To estimate the rate of delivery of the nonpresenting twin first and to identify risk factors for such an event by using a cohort of opposite-sex twins for whom the intrauterine order was well documented with ultrasonography before delivery.

Methods: We conducted a retrospective cohort study of all opposite-sex dichorionic twins in a single tertiary center between 2002 and 2016. Reports of ultrasonograms performed less than 2 weeks before birth were reviewed for information on twins' presenting order in relation to fetal sex. Intrauterine labeling was compared with labeling at the time of birth. Multivariable regression analysis was used to identify factors associated with delivery of the nonpresenting twin first.

Results: Of 1,746 women with dichorionic twin pregnancies, 942 (53.9%) had opposite-sex twins and 617 had recent data on ultrasonographic twin labeling. In 456 of 617 (73.9%) pregnancies, both twins were delivered by cesarean delivery and in 161 of 617 (26.1%) pregnancies, one or both twins were delivered vaginally. The overall rate of delivery of the nonpresenting twin first was 6.8% (95% CI 4.5-9.1%, 31/456) in the cesarean group; there were no deliveries (95% CI 0.0-2.3%, 0/161) of the nonpresenting twin first in the vaginal group (P=.001). The following factors were independently associated with delivery of the nonpresenting twin first at cesarean delivery: discordance greater than 25% in birth weights (17.5%, adjusted odds ratio [OR] 4.0, 95% CI 1.7-9.1), nonvertex presentation of the presenting twin (11.6%, adjusted OR 3.8, 95% CI 1.7-8.3), and gestational age less than 32 0/7 weeks (14.9%, adjusted OR 3.6, 95% CI 1.6-7.8).

Conclusions: Delivery of the nonpresenting twin first in dichorionic, opposite-sex twins at the time of birth occurs in 6.8% of cesarean deliveries. Clinicians and researchers should be aware of this phenomenon because it has implications for neonatal care and long-term outcome.