How many embryos should be transferred in couples undergoing ART?
Multiple pregnancy creates serious health risks for the mother (such as premature labour, diabetes and high blood pressure) and for the babies, who are at much higher risk than single babies of problems including premature birth, low birth weight, cerebral palsy and perinatal death. Single embryo transfer is now being seriously considered in order to reduce multiple pregnancies but this needs to be balanced against the risk of lowering the overall live birth rate. Researchers in The Cochrane Collaboration reviewed the evidence about the number of embryos transferred in women undergoing ART. The search is current to July 2013.
We found 14 randomised controlled trials with a total of 2165 participants. Most were not commercially funded.
Double versus repeated single embryo transfer
Based on low quality evidence, there was no indication that overall live birth rates differed substantially when repeated single embryo transfer (either two cycles of single embryo transfer or one cycle of single embryo transfer followed by transfer of a single frozen embryo in a natural or hormone-stimulated cycle) was compared with double embryo transfer. The evidence suggested that for a woman with a 42% chance of live birth following a single cycle of double embryo transfer, the chance following repeated single embryo transfer would be between 31% and 44%. The risk of multiple birth was very much lower in the single embryo transfer group: for a woman with a 13% risk of multiple pregnancy following a single cycle of double embryo transfer, the estimated risk following a repeated single transfer was between 0% and 2%.
Double versus single embryo transfer
We found high quality evidence that the chances of live birth were lower after one cycle of fresh single embryo transfer than after one cycle of fresh double embryo transfer. For a woman with a 45% chance of live birth following a single cycle of double embryo transfer, the chance following a single cycle of single embryo transfer was between 24% and 33%. However, the risk of twins was about seven times higher after double embryo transfer.
Repeated single embryo transfer appears the best option for most women undergoing ART. Most of the evidence currently available concerns younger women with a good prognosis.