title: Evaluation of the twin peak or lambda sign in determining chorionicity in multiple pregnancy.
abstract: OBJECTIVE
  To assess prospectively the diagnostic capabilities of ultrasound assessment, using the twin peak or lambda sign, in determining chorionicity in multiple pregnancy.
  
  
  METHODS
  Fifty-five cases of multiple pregnancy were assessed. Real-time ultrasound scans of the origin of the inter-twin membrane for the presence or absence of the twin peak/lambda sign were performed and permanent images recorded. Chorionicity was determined by placental pathologic analysis.
  
  
  RESULTS
  Presence or absence of the twin peak/lambda sign as determined by real-time ultrasound correctly predicted chorionicity in 34 of 36 dichorionic and seven of eight monochorionic twin pregnancies: sensitivity for dichorionicity 94%, specificity 88%, positive predictive value 97%, and negative predictive value 78%. Real-time assessment was superior to analysis of permanent films.
  
  
  CONCLUSIONS
  Ultrasound assessment of chorionicity using the twin peak/lambda sign has high sensitivity and specificity, but accuracy may not be sufficient to guide clinical management in all cases.
authors.affiliation: Department of Obstetrics and Gynecology, University of Calgary, Canada.
authors.forename: S L
authors.initials: SL
authors.lastname: Wood
authors.forename: R
authors.initials: R
authors.lastname: St Onge
authors.forename: G
authors.initials: G
authors.lastname: Connors
authors.forename: P D
authors.initials: PD
authors.lastname: Elliot
mesh_terms: D002823:Chorion / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D011237:Predictive Value of Tests; D011247:Pregnancy; D011272:Pregnancy, Multiple; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal