title: Sonographic determination of first trimester twin chorionicity and amnionicity.
abstract: OBJECTIVE
  To determine the reliability of first trimester sonography in the detection of twin chorionicity and amnionicity.
  
  
  METHODS
  Sets of twins (179) were evaluated sonographically in the first trimester. Chorionicity was determined by identifying the number of gestational sacs. The presence or absence of an amnion about each embryo/fetus was also recorded. The placentas for each case were evaluated to determine chorionicity and amnionicity.
  
  
  RESULTS
  Of the 179 twin pairs, 158 were dichorionic/diamniotic; 19 were monochorionic/diamniotic, and two pairs were monochorionic/monoamniotic. Chorionicity and amnionicity were correctly determined in each case.
  
  
  CONCLUSIONS
  The chorionicity of twin pregnancies can be determined from the time that the gestational sacs are visualized within the uterus. Transvaginal sonography can reliably determine the amnionicity of monochorionic twin pregnancies after 7 weeks, menstrual age.
authors.affiliation: Department of Obstetrics, Gynecology and Reproductive Sciences, University of Pittsburgh School of Medicine, Pennsylvania, USA.
authors.forename: L M
authors.initials: LM
authors.lastname: Hill
authors.forename: P
authors.initials: P
authors.lastname: Chenevey
authors.forename: J
authors.initials: J
authors.lastname: Hecker
authors.forename: J G
authors.initials: JG
authors.lastname: Martin
mesh_terms: D000650:Amnion / Q000000981:diagnostic imaging; D002823:Chorion / Q000000981:diagnostic imaging*; D019160:Endosonography; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011272:Pregnancy, Multiple / Q000502:physiology*; D014427:Twins; D016216:Ultrasonography, Prenatal