title: Twin reversed arterial perfusion sequence is more common than generally accepted.
abstract: BACKGROUND
  Approximately 75% of monozygotic twin pregnancies share one monochorionic placenta where placental anastomoses are virtually always present to connect the two fetoplacental circulations. These anastomoses cause several serious complications such as acardiac twinning. Acardiac twins lack a functional heart but nevertheless show fetal growth because the normal pump twin perfuses the acardiac body through arterioarterial (AA) and venovenous (VV) anastomoses. The widely accepted 1% monochorionic acardiac incidence dates back to 1944 and the associated 1:35,000 pregnancies to 1953. Our aim was to update this analysis.
  
  
  METHODS
  We accepted the 1% (actually 1.1%) monochorionic acardiac incidence due to lack of more precise data, included the recently observed 58% early cessation of acardiac development as well as consequences of assisted reproductive technology, and assessed the incidence of acardiac twinning under conditions of AA-VV anastomoses.
  
  
  RESULTS
  Early acardiac monochorionic twinning increased from 1.1% to 1.1/(1-0.58) = 2.6%, from 1:35,000 to 1:9,500 to 11,000 pregnancies, depending on number and method of assisted reproductive technology, and occurs in approximately 1:8 AA-VV anastomoses-containing monochorionic placentas.
  
  
  CONCLUSIONS
  Early acardiac twinning is not a rare event. The 1944-based 1% acardiac monochorionic incidence has a weak basis and could therefore be (much) larger. Knowing this incidence more precisely may contribute to our knowledge of embryonic splitting in unequal cell masses.
authors.affiliation: Department of Biomedical Engineering & Physics, Academic Medical Center, Amsterdam, The Netherlands.
authors.forename: Martin J C
authors.initials: MJ
authors.lastname: van Gemert
authors.affiliation: Department of Biomedical Engineering & Physics, Academic Medical Center, Amsterdam, The Netherlands.|Department of Clinical Chemistry and Laboratory Medicine, Leiden University Medical Center, Leiden, The Netherlands.
authors.forename: Jeroen P H M
authors.initials: JP
authors.lastname: van den Wijngaard
authors.affiliation: Department of Obstetrics and Gynecology, Radboud University Medical Center, Nijmegen, The Netherlands.
authors.forename: Frank P H A
authors.initials: FP
authors.lastname: Vandenbussche
mesh_terms: D005260:Female; D005333:Fetus / Q000098:blood supply*; D006801:Humans; D010920:Placenta / Q000098:blood supply*; D011247:Pregnancy; D014430:Twins, Monozygotic
keywords: acardiac incidence; acardiac incidence in arterioarterial-venovenous anastomoses containing placentas; acardiac twinning is not rare; acardiac twins; twin-reversed arterial perfusion sequence