Perinatal Switch In Twin Order Between The Last Prenatal Ultrasound And Birth Tiran Dias, Francesco Antonio, Samantha Ladd, Amar Bhide, Aris Papageorghiou, Basky ThilaganathanSt George’s University of London, Crammer Terrace SW17 0RE, UK. Objectives Methods Results Conclusions • The twin ultrasound database from the Southwest Thames Obstetric Research Collaborative (STORK) was used to identify all examination carried out within four weeks of birth. • The perinatal switch was evaluated by matching discrepancies in ultrasound estimated fetal weight (EFW) with birth weight discordance. • The twin perinatal switch rate estimated from twin sizes was confirmed by comparing pre and post-natal order in discordant sex twins. • A total of 2103 twin pairs with ultrasound EFW and birth weights were assessed. • The anticipated birth order according to the ultrasound scan changed in 753 (35.8%) twin deliveries. • The perinatal switch in discordant sex twins 66/156 (42.3%) was not significantly different (p=0.12). • It is often assumed by obstetricians, neonatologists and parents alike that the prenatal nomenclature used to identify twins on ultrasound is equally applicable after their delivery. • The aim of this study is to use a large regional twin ultrasound database to validate the effectiveness of a scan just prior to birth in predicting twin birth order. • Approximately one-third of twin pregnancies switch order between the last scan and birth determined by size discordance. • This should be borne in mind when delivering twins discordant for anomalies. References Dias T, Ladd S, Mahsud-Dornan S, Bhide A, Papageorghiou AT, Thilaganathan B. Systematic labeling of twin pregnancies on ultrasound. Ultrasound Obstet Gynecol. 2011; 38: 130-3. Dias T, Contro E, Thilaganathan B, Khan H, Zanardini C, Mahsud-Dornan S, Bhide A. Pregnancy outcome of monochorionic twins: does amnionicity matter? Twin Res Hum Genet. 2011; 14: 586-92.