title: First-trimester ultrasound determination of chorionicity in twin pregnancy.
abstract: OBJECTIVE
  To determine the accuracy of ultrasound at 11-14 weeks' gestation in the diagnosis of chorionicity in twin pregnancy.
  
  
  METHODS
  This was a retrospective observational study of data obtained between 1999 and 2010. At the first-trimester routine ultrasound scan, chorionicity was assigned according to the number of placental masses and T or λ-signs for a single placental mass. Chorionicity was confirmed by histology or discordant sex at birth.
  
  
  RESULTS
  A total of 648 pregnancies were assigned chorionicity by first-trimester ultrasound during the study period. Chorionicity was ascertained in 613 cases, either by histology (n = 340) or discordant sex (n = 273). Chorionicity was correctly assigned by ultrasound at 11-14 weeks in 612 of 613 pregnancies (accuracy 99.8%). Sensitivity and specificity for determining monochorionicity were 100% and 99.8%, respectively.
  
  
  CONCLUSIONS
  First-trimester ultrasound can be used to determine chorionicity reliably by noting the number of placental masses and T or λ-signs. Determination of twin chorionicity is important and should be completed in the first trimester.
authors.affiliation: Fetal Medicine Unit, St George's University of London, London, UK.
authors.forename: T
authors.initials: T
authors.lastname: Dias
authors.forename: T
authors.initials: T
authors.lastname: Arcangeli
authors.forename: A
authors.initials: A
authors.lastname: Bhide
authors.forename: R
authors.initials: R
authors.lastname: Napolitano
authors.forename: S
authors.initials: S
authors.lastname: Mahsud-Dornan
authors.forename: B
authors.initials: B
authors.lastname: Thilaganathan
mesh_terms: D002823:Chorion / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D010920:Placenta / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D059285:Pregnancy, Twin; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000379:methods