Not all twins are monozygotic after elective single embryo transfer: analysis of 32,600 elective single embryo transfer cycles as reported to the Society for Assisted Reproductive Technology

Elective single ET improves, but does not completely eliminate the risk of multiple pregnancies. Patients should be counseled that there might be up to a ∼2% risk of multiple pregnancies, of which up to 18% can be dizygotic. Patients with elevated BMI and unexplained fertility may be at higher risk for sex discordant multiple pregnancies and patients undergoing fresh cycles may be at higher risk for sex concordant multiple pregnancies.