title: A review of the mechanisms and evidence for typical and atypical twinning.
abstract: The mechanisms responsible for twinning and disorders of twin gestations have been the subject of considerable interest by physicians and scientists, and cases of atypical twinning have called for a reexamination of the fundamental theories invoked to explain twin gestations. This article presents a review of the literature focusing on twinning and atypical twinning with an emphasis on the phenomena of chimeric twins, phenotypically discordant monozygotic twins, mirror-image twins, polar body twins, complete hydatidiform mole with a coexistent twin, vanishing twins, fetus papyraceus, fetus in fetu, superfetation, and superfecundation. The traditional models attributing monozygotic twinning to a fission event, and more recent models describing monozygotic twinning as a fusion event, are critically reviewed. Ethical restrictions on scientific experimentation with human embryos and the rarity of cases of atypical twinning have limited opportunities to elucidate the exact mechanisms by which these phenomena occur. Refinements in the modeling of early embryonic development in twin pregnancies may have significant clinical implications. The article includes a series of figures to illustrate the phenomena described.
authors.affiliation: Department of Maternal-Fetal Medicine, Royal Womens Hospital, Melbourne, Victoria, Australia.
authors.forename: Helen C
authors.initials: HC
authors.lastname: McNamara
authors.affiliation: Department of Maternal-Fetal Medicine, Royal Womens Hospital, Melbourne, Victoria, Australia; Department of Obstetrics and Gynaecology, University of Melbourne, Melbourne, Victoria, Australia.
authors.forename: Stefan C
authors.initials: SC
authors.lastname: Kane
authors.affiliation: Murdoch Childrens Research Institute, Royal Children's Hospital, Melbourne, Victoria, Australia.
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Craig
authors.affiliation: Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, Melbourne, Victoria, Australia.
authors.forename: Roger V
authors.initials: RV
authors.lastname: Short
authors.affiliation: Department of Maternal-Fetal Medicine, Royal Womens Hospital, Melbourne, Victoria, Australia; Department of Obstetrics and Gynaecology, University of Melbourne, Melbourne, Victoria, Australia. Electronic address: mark.umstad@thewomens.org.au.
authors.forename: Mark P
authors.initials: MP
authors.lastname: Umstad
mesh_terms: D000650:Amnion / Q000196:embryology*; D002823:Chorion / Q000196:embryology*; D047108:Embryonic Development; D005260:Female; D005333:Fetus; D006801:Humans; D006828:Hydatidiform Mole; D009028:Morula; D011247:Pregnancy; D011248:Pregnancy Complications; D059285:Pregnancy, Twin / Q000502:physiology*; D027724:Reproductive Techniques, Assisted; D013476:Superfetation; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D014594:Uterine Neoplasms
keywords: assisted reproductive technologies; chimeric twins; complete hydatidiform mole with coexistent twin; dizygotic; fetus in fetu; fetus papyraceus; mirror-image twins; monozygotic; phenotypically discordant monozygotic twins; polar body twins; superfecundation; superfetation; twins; vanishing twin