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Monozygotic twinning associated with assisted reproductive technologies: a review

¹ Andrology and IVF Laboratories, Departments of² Surgery³ , Physiology⁴ Obstetrics and Gynecology, University of Utah School of Medicine, Salt Lake City, Utah 84108, USA

Correspondence should be addressed to D T Carrell at Andrology and IVF Laboratories, University of Utah School of Medicine, 675 Arapeen Drive, Ste 205, Salt Lake City, Utah 84117, USA; Email: douglas.carrell{at}hsc.utah.edu

Twin birth rates have increased markedly in developed countries since the 1970s for two primary reasons: increasing maternal age and the advent and increasing use of fertility treatments. In addition, monozygotic (MZ) twin pregnancies have been reported to occur at a significantly higher rate following assisted reproductive technologies (ART) procedures compared with the natural incidence. Twin pregnancies are of concern due to a dramatically increased risk of associated complications. Monozygotic twin pregnancies carry a 10–20% risk of twin–twin transfusion syndrome, and monoamniotic monochorionic twins are additionally at risk for cord entanglement. While the mechanisms and contributory factors for dizygotic twinning are well established, very little is known about the mechanisms involved in MZ twinning or the factors that contribute to its occurrence. In this review, we will discuss a number of potential mechanisms involved in MZ twinning and explore factors that may be contributing to the increased incidence of ART-associated MZ twins. An improved understanding of the factors that contribute to increased MZ twinning associated with ART will help to elucidate the poorly understood mechanisms involved in the process and will further aid in reducing the overall incidence of multiple pregnancies with their associated risks following ART procedures.