Recent advances in embryo culture and transfer techniques have resulted in improved IVF pregnancy rates and reduced multiple birth rates. Traditionally, embryos are transferred back to the uterus on post-retrieval day 3, so-called “a day-3 transfer”. On this day is not uncommon to transfer three or four embryos. However, it is now possible to extend the growth of the embryos in the laboratory until they become blastocysts. A blastocyst is a stage of embryonic development, which usually occurs on day five after fertilization, when the embryo has 50-200 cells. Blastocysts are more capable of readily implanting into the uterus, once they are transferred. Usually the healthiest, strongest embryos make it to the blastocyst stage.
The extended embryo culture allows the selection of the best possible embryos for transfer. The selection process translates into the transfer of fewer embryos back into the uterus. The transfer of fewer number of embryos result in lowering the risk of high order multiple births while maintaining high pregnancy rates. In fact, nowadays there is a trend to transfer a single embryo in the blastocyst stage in selected patient populations.