Controversies in OB/GYN: Is single embryo transfer okay in women under 38 undergoing their first IVF cycle?

Key Points

Yes. Single embryo transfer can yield satisfactory delivery rates-but at a price.

By Christina Bergh, MD, PhD

Not surprisingly, the most important factor influencing the multiple birthrate is the number of embryos transferred. Obviously, transferring only one embryo would result mainly in singletons but might also result in a considerable decline in the overall birthrate. Randomized studies, as well as a recent Cochrane review comparing single (SET) versus double embryo transfer (DET) in good prognosis women, reached the same conclusion: Compared to DET, SET reduces the multiple birthrate to a minimum, but also results in statistically and clinically significant lower pregnancy and live birthrates.^1,5 Similar results were found when a Dutch study randomized women having a less favorable prognosis.

The largest randomized controlled trial (RCT) so far, however, found no substantial reduction in the live birthrate if the SET in the fresh cycle was combined with a SET frozen/thawed cycle from the same ovarian stimulation, if live birth was not achieved in the fresh cycle.⁶ Thus, the cumulative live birthrate stayed fairly constant, which is of course of paramount importance to patients.

Observational studies comparing transfers of one vs. two embryos

In recent years, several observational studies, mainly from the Nordic countries, Belgium, and the Netherlands, have compared the outcome after two versus one embryo transfer.¹ Similar pregnancy and delivery rates (just above 30% and almost 30%) were achieved when transferring one or two embryos. The multiple birthrate fell dramatically in the SET group while it stayed high in the DET group. The reason for the similar results in the SET and DET groups is of course that the two groups are not strictly comparable, in that the good prognosis patients received one embryo while the poor prognosis women received two. Possibly, the overall pregnancy and delivery rates would have been higher if all women had received two embryos, but at the price of a high multiple birthrate.

Multiple births fell from 25% to 5%!

Perhaps more interesting, however, are recently published Swedish national registry data for the years up to 2004.⁷ Swedish laws began to change in parallel with the RCTs. Since 2003, the rules from the National Board of Health and Welfare call for transferring one embryo as the normal routine, and transferring two embryos only when there seems to be a low risk of having twins. Because yearly reporting to this registry is compulsory, the cover rate for the registry is thus 100%.