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.

Objective: To study the incidence and risk factors of multiple pregnancies after elective single ET.

Design: Historic cohort.

Setting: Not applicable.

Patient(s): Women <35 years of age undergoing elective single ET entered in the SART CORS database from 2010 to 2013.

Interventions: Not applicable.

Main Outcome Measure(s): Rate of sex discordant pregnancies. Rate of same sex pregnancies and risk factors for both same sex and sex discordant pregnancies.

Result(s): A total of 32,600 cycles were reported to SART CORS during this time period. There were 15,143 pregnancies from which 14,888 were singletons (98.3%), 23 sex discordant (0.15%) multiple pregnancies, 226 (1.5%) sex concordant multiple pregnancies, and 6 (0.01%) pregnancies without sex information. When Weinberg's differential rule was applied, the rate of dizygotic pregnancies was calculated to be 18%. Unexplained infertility was found to be the biggest risk factor for sex discordant multiple pregnancies (adjusted odds ratio 4.33, 95% confidence interval 1.4-13.1), followed by elevated body mass index (BMI). The only risk factor found for sex concordant pregnancies was undergoing a fresh transfer (adjusted odds ratio 1.4, 95% confidence interval 1.02-1.95).

Conclusion(s): 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.