Incidence and zygosity of twin births following transfers using a single fresh or frozen embryo

Are all twin births following single embryo transfer (SET) monozygotic? Between 1 in 10 and in 1 in 5 twins born after SET are the result of a concurrent natural conception. The twinning rate after SET is higher than following natural conception. Most studies of twins following SET have incorrectly assumed monozygosity or have not been able to assess the zygosity. This study is a retrospective cohort study assessing the gender discordance of all live born twins following fresh or frozen SET. A total of 4701 patients in a large private IVF unit who gave birth following SET with a fresh or frozen embryo with complete follow-up. Of 137 viable twins at the 7-week ultrasound, 109 were delivered as twins. Gender discordance and Weinberg's differential rule were used to estimate dizygosity. Twin rates were compared for fresh and frozen transfers by insemination method and transfer day. The overall live twin birth rate was 2.3% (109/4701). Based on the 7-week scan, 2 of the twins were monochorionic monoamniotic, 62 were monochorionic diamniotic and 45 were dichorionic diamniotic. There were a total of 12 gender discordant twins (11%), 7 from the Day 2/3 transfers and 5 from Day 5 transfers. Nine of the 12 discordant twins were from natural cycle frozen embryo transfers, the remaining 3 were from fresh cycles. To assess gender discordance only live born twins were studied. DNA fingerprinting of twins is a more accurate way to assess zygosity than measuring gender discordance. Same sex twins in this study are not necessarily monozygotic and the dizygotic rate in this study may therefore be higher. This rate was estimated using Weinberg's differential rule. As many as 1 in 5 twins born after SET may be the result of a concurrent natural conception. Couples therefore need to be counselled regarding the relative benefits and risks of intercourse in assisted reproduction technology cycles where spontaneous conception is possible. None. Not applicable.