title: Heterokaryotypic pregnancy: monozygotic monochorionic twins discordant for trisomy 13.
abstract: BACKGROUND
  The occurrence of a discordant chromosomal abnormality in monozygotic twins is an extremely rare condition.
  
  
  METHODS
  We report the prenatal sonographic findings and cytogenetic studies in a monochorionic twin pregnancy discordant for severe fetal anomalies. Amniocenteses demonstrated heterokaryotypia for trisomy 13 in monozygotic twins. The pregnancy was managed conservatively, resulting in the delivery of discordant twins at 32 weeks. The structurally normal twin survived without sequelae, but the abnormal twin died shortly after delivery.
  
  
  CONCLUSIONS
  This report adds to the literature the second known case of a spontaneously conceived monochorionic twin pregnancy discordant for trisomy 13 and highlights the necessity of sampling both fetuses in cases of monochorionic twins presenting with discordant structural anomalies.
authors.affiliation: Maternal-Fetal Medicine Unit, Department of Obstetrics and Gynecology, San Jose Hospital, University of Santiago de Chile, Casilla, Santiago, Chile. fetalmed@yahoo.com
authors.forename: Waldo
authors.initials: W
authors.lastname: Sepulveda
authors.forename: Amy E
authors.initials: AE
authors.lastname: Wong
authors.forename: Myriam
authors.initials: M
authors.lastname: Ocaranza
mesh_terms: D000328:Adult; D000649:Amniocentesis; D025063:Chromosome Disorders / Q000175:diagnosis / Q000000981:diagnostic imaging / Q000235:genetics; D002882:Chromosomes, Human, Pair 13 / Q000000981:diagnostic imaging / Q000235:genetics; D004200:Diseases in Twins / Q000175:diagnosis* / Q000000981:diagnostic imaging / Q000235:genetics; D005260:Female; D006801:Humans; D011247:Pregnancy; D014314:Trisomy / Q000175:diagnosis / Q000235:genetics; D000073839:Trisomy 13 Syndrome; D014430:Twins, Monozygotic / Q000235:genetics*; D014463:Ultrasonography