title: Ultrasound determination of chorionicity and perinatal outcome in twin pregnancies using dividing membrane thickness.
abstract: OBJECTIVE
  To assess the value of ultrasound measurement of twin dividing membrane thickness in predicting chorionicity and perinatal outcome. To compare magnified with unmagnified images, and measurements taken with dividing membranes parallel and perpendicular to the ultrasound beam.
  
  
  METHODS
  This was a prospective, observational cohort study of twin-pregnancy ultrasound examinations during the second and third trimesters with chorionicity confirmed by a pathologist. Ultrasound measurements of the dividing membrane thickness were recorded in 44 twin pregnancies using unmagnified and magnified images. Measurements were taken at various membrane sites either horizontal or vertical to the ultrasound beam, depending on the orientation of the dividing membrane. We compared the mean membrane thickness between monochorionic and dichorionic pregnancies as well as between pregnancies with good and adverse perinatal outcome.
  
  
  RESULTS
  Membranes were significantly thicker in dichorionic pregnancies when compared with the monochorionic pregnancies (2.35 vs. 1.69 mm, p = 0.021). A cut-off point of >/=2.0 mm provided a sensitivity of 75.7%, a specificity of 85.7%, a positive predictive value of 96.6%, and a negative predictive value of 40.0%, for determining dichorionicity. Two of 7 monochorionic and 24 of 37 dichorionic pregnancies (59.1%) had good perinatal outcome. These pregnancies had significantly thicker membranes (2.44 vs. 1.96 mm, p = 0.022). Even among the dichorionic pregnancies, a membrane thicker than 2.2 mm was associated with a significantly higher proportion with good perinatal outcome (80.0 vs. 47.1%, relative risk 1.7, p = 0.036). Magnified images obtained with the ultrasound beam parallel to the membrane identified adverse outcome more accurately than unmagnified and perpendicular images. A cut-off point of >2.2 mm for membrane thickness was found to be the best predictor of good outcome.
  
  
  CONCLUSIONS
  Membrane thickness can be used to identify chorionicity and adverse perinatal outcome, particularly if the ultrasound images are magnified and parallel to the ultrasound beam.
authors.affiliation: Department of Obstetrics and Gynecology, Westchester Medical Center, Valhalla, N.Y., USA. Bracero@aol.com
authors.forename: Luis A
authors.initials: LA
authors.lastname: Bracero
authors.forename: Daniel W
authors.initials: DW
authors.lastname: Byrne
mesh_terms: D000328:Adult; D002823:Chorion / Q000000981:diagnostic imaging* / Q000502:physiology; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D015588:Observer Variation; D011237:Predictive Value of Tests; D011247:Pregnancy; D011256:Pregnancy Outcome; D011264:Pregnancy Trimesters; D011272:Pregnancy, Multiple / Q000502:physiology*; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D014427:Twins; D016216:Ultrasonography, Prenatal / Q000379:methods*