title: Prediction of chorionicity in twin pregnancies at 10-14 weeks of gestation.
abstract: OBJECTIVE
  To examine the accuracy of sonographic determination of chorionicity in twin pregnancies at 10-14 weeks of gestation.
  
  
  METHODS
  Prospective study on the sonographic prediction of chorionicity at 10-14 weeks of gestation.
  
  
  METHODS
  During a 30 month period, from October 1997 to May 2000, 165 women attending the departments of fetal medicine or ultrasound.
  
  
  METHODS
  Sonographic criteria used in the diagnosis of chorionicity were the number of placental sites, the lambda (lambda) and T signs and the thickness of the inter-twin membrane. The diagnosis of chorionicity was made at the time of the ultrasound examination using all these features and subsequently compared with the postnatal diagnosis, confirmed either by placental histology or discordancy in infant sex.
  
  
  RESULTS
  In 150 cases with confirmation of chorionicity following delivery, 116 were postnatally classified as dichorionic and 34 monochorionic. Prenatal ultrasound examination correctly identified chorionicity in 149 (99.3%) cases. The most reliable indicator for dichorionicity was a combination using the lambda sign or two separate placentae with a sensitivity and specificity of 97.4% and 100%, respectively. The most useful test in predicting monochorionicity was the T sign with a sensitivity of 100% and specificity of 98.2%. Measurement of the inter-twin membrane thickness was a less reliable indicator where the sensitivity for dichorionicity and specificity for monochorionicity was only 92.6%.
  
  
  CONCLUSIONS
  Ultrasound examination of twin pregnancies at 10-14 weeks of gestation predicts chorionicity with a high degree of accuracy using a combination of the number of placentae, lambda and T signs and inter-twin membrane thickness. All hospitals should encourage departments providing ultrasound services to undertake chorionicity determination when examining women with twin pregnancies at this gestation.
authors.affiliation: National Maternity Hospital, Dublin, Ireland.
authors.forename: Stephen G M
authors.initials: SG
authors.lastname: Carroll
authors.forename: Peter W
authors.initials: PW
authors.lastname: Soothill
authors.forename: Sherif A
authors.initials: SA
authors.lastname: Abdel-Fattah
authors.forename: Helen
authors.initials: H
authors.lastname: Porter
authors.forename: Imogen
authors.initials: I
authors.lastname: Montague
authors.forename: Phillipa M
authors.initials: PM
authors.lastname: Kyle
mesh_terms: D002823:Chorion / Q000033:anatomy & histology* / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D010920:Placenta / Q000000981:diagnostic imaging; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011272:Pregnancy, Multiple; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D014427:Twins; D016216:Ultrasonography, Prenatal / Q000379:methods / Q000592:standards