title: Re-Thinking Elective Single Embryo Transfer: Increased Risk of Monochorionic Twinning - A Systematic Review.
abstract: OBJECTIVE
  Multiple pregnancies have tripled in the United States over the past 3 decades. Attributed to increasing maternal age at delivery but more so assisted reproductive technological advances, an effort has been made to decrease twinning through elective single embryo transfer. We sought to review and evaluate risks of monochorionic twinning as a predictable consequence of increasing utilization of elective single embryo transfer on perinatal outcomes. Primary outcomes included twinning rates, fetal anomalies, growth, preterm birth, and mortality. Secondary outcomes included neurological and pulmonary disability, intrauterine growth restriction, and congenital cardiac anomalies and twin-twin transfusion syndrome.
  
  
  METHODS
  PubMed and Embase.
  
  
  RESULTS
  A total of 106 studies identified by systematic search met the inclusion criteria. The trend for lower numbers of embryos transferred has inadvertently led to an increase in monochorionic twinning. This is associated with worse outcomes compared to dichorionic twinning and singleton gestations for all outcomes studied.
  
  
  CONCLUSIONS
  Of great concern for monochorionic twins is the risk profile of significant morbidity and mortality. Transfer of 2 embryos should be considered to avoid higher risks inherent to the shared placental phenomena related to monochorionic twins.
authors.affiliation: Division of Maternal Fetal Medicine, Department of Obstetrics &amp; Gynecology, St. Barnabas Medical Center Livingston, Livingston, NJ, USA.
authors.forename: Margaret
authors.initials: M
authors.lastname: Dziadosz
authors.forename: Mark I
authors.initials: MI
authors.lastname: Evans
mesh_terms: D005260:Female; D005315:Fetal Diseases / Q000209:etiology* / Q000517:prevention & control; D006801:Humans; D007234:Infant, Premature; D011247:Pregnancy; D011256:Pregnancy Outcome; D059285:Pregnancy, Twin; D047928:Premature Birth / Q000209:etiology* / Q000517:prevention & control; D056826:Single Embryo Transfer / Q000009:adverse effects*; D014430:Twins, Monozygotic
keywords: Assisted reproductive therapy; Dichorionic twins; In vitro fertilization; Monochorionic twins; Risk; Single embryo transfer