title: Is zygosity or chorionicity the main determinant of fetal outcome in twin pregnancies?
abstract: OBJECTIVE
  The purpose of this study was to examine whether fetal outcome in twin pregnancies is dependent on zygosity or chorionicity.
  
  
  METHODS
  This was a prospective observational study comprised of women with twin pregnancies who attended the fetal medicine unit at St Michael's Hospital, Bristol, Ireland, during the years 1998 to 2000 and who were delivered in hospitals in south west England. After delivery, zygosity was determined with umbilical cord blood with the use of microsatellite markers that were amplified by polymerase chain reaction. Placentae were examined histologically for chorionic type. The perinatal outcomes of 3 groups of monozygotic monochorionic, monozygotic dichorionic, and dizygotic pregnancies were compared with the use of the Mann-Whitney U test and the Fisher's exact test.
  
  
  RESULTS
  All 92 dizygotic and 15 monozygotic dichorionic pregnancies resulted in live births. In 7 of the 39 cases in the monozygotic monochorionic group, either both twins were not live born or delivery occurred <24 weeks of gestation. The gestational age at delivery and birth weight were significantly lower, and there were a greater number of cases with birth weight discordancy of >25% in the monochorionic pregnancies compared with the other 2 groups (P < .05). There were no significant differences in any of the study parameters between the monozygotic dichorionic and dizygotic groups.
  
  
  CONCLUSIONS
  Fetal outcome in twin pregnancies is related to chorionicity rather than zygosity.
authors.affiliation: St Michael's Hospital, Bristol, United Kingdom.
authors.forename: Stephen G M
authors.initials: SG
authors.lastname: Carroll
authors.forename: Linda
authors.initials: L
authors.lastname: Tyfield
authors.forename: Louise
authors.initials: L
authors.lastname: Reeve
authors.forename: Helen
authors.initials: H
authors.lastname: Porter
authors.forename: Peter
authors.initials: P
authors.lastname: Soothill
authors.forename: Phillipa Mm
authors.initials: PM
authors.lastname: Kyle
mesh_terms: D002823:Chorion; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D010920:Placenta / Q000033:anatomy & histology; D011247:Pregnancy; D011256:Pregnancy Outcome; D011272:Pregnancy, Multiple; D011446:Prospective Studies; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D015053:Zygote