title: Confined blood chimerism in monochorionic dizygous (MCDZ) twins.
abstract: OBJECTIVE
  To present the risk factors, prenatal and postnatal implications of monochorionic dyzygotic twinning.
  
  
  METHODS
  We present a case of monochorionic twins diagnosed on first-trimester ultrasound following a 2-embryo transfer in vitro fertilisation pregnancy. Postnatal buccal smears performed as part of a twin research study revealed the twins to be dizygous, and DNA testing on peripheral blood lymphocytes confirmed the presence of blood chimerism.
  
  
  RESULTS
  Despite the monochorionic placentation, buccal swabs revealed discordant results for all four microsatellite loci tested, consistent with dizygosity. Zygosity testing on peripheral blood lymphocytes revealed that, of the nine DNA microsatellite loci tested, three had all four parental alleles represented. This result was concordant between the twins. The diagnosis of blood chimerism was thus confirmed, presumably due to the inter-twin vascular anastomoses of the monochorionic placenta.
  
  
  CONCLUSIONS
  Monochorionic dizygous twinning is rare, but appears to be more common after assisted reproduction. It is presumed that outer cell mass fusion may occur when two embryos are replaced in close proximity. These pregnancies are not only at risk for the usual complications of monochorionicity, but also have the potential to be heterokaryotypic. Postnatally, the twins may have long-term blood chimerism.
authors.affiliation: Perinatal Medicine, Mercy Hospital for Women, 163 Studley Road, Heidelberg 3084, Australia. spwalker@unimelb.edu.au
authors.forename: Susan P
authors.initials: SP
authors.lastname: Walker
authors.forename: Simon
authors.initials: S
authors.lastname: Meagher
authors.forename: Susan M
authors.initials: SM
authors.lastname: White
mesh_terms: D046528:Chimerism; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D007223:Infant; D008297:Male; D009061:Mouth Mucosa; D011247:Pregnancy; D011272:Pregnancy, Multiple; D014429:Twins, Dizygotic / Q000097:blood* / Q000235:genetics; D016216:Ultrasonography, Prenatal