Re-Thinking Elective Single Embryo Transfer: Increased Risk of Monochorionic Twinning - A Systematic Review.

BACKGROUND/OBJECTIVES 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. DATA SOURCES 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. DISCUSSION 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.