Factors affecting fetal presentation in twin pregnancies across gestation.

To describe pregnancy characteristics associated with the occurrence of spontaneous version in twin pregnancies from 20 weeks until delivery. Review of ultrasound and pregnancy data for all twin pregnancies delivered >24 weeks by one Maternal-Fetal Medicine practice from June 2005-May 2012. For each 4-week gestational age window, fetal presentations were recorded, as well as the likelihood of the final presentation being vertex for Twin A and vertex for both twins. Case-control analysis was performed to estimate associations between pregnancy characteristics and spontaneous version of Twin A. A total of 491 twin pregnancies were included. The distribution of fetal presentations changed significantly from 20 weeks to delivery, but the presentation at each gestational age interval was significantly associated with the final position of Twin A and the likelihood of vertex-vertex presentation at delivery. The likelihood of spontaneous version of Twin A decreased from 27.9% after 24-27 6/7 weeks, to 18.8% after 28-31 6/7 weeks, to 8.2% after 32-35 6/7 weeks. Pregnancy characteristics associated with spontaneous version of Twin A were a prior vaginal delivery and increased fetal size of either twin. The likelihood of spontaneous version of Twin A decreases as pregnancy advances. Parity and increased fetal size are associated with spontaneous version.