Blastocyst or day five embryo transfer is an exciting new advance in IVF therapy. Since the birth of IVF technology in the 1970’s, IVF has involved the transfer of embryos at a relatively early stage of the egg retrieval. Embryos had generally reached the 2-4 cell stage of development at that time.
Beginning in the late 1980’s-early 1990’s, we started culturing the embryos a little longer, to the third day after egg retrieval, when we would expect the embryos to be about 8 cell stage development. The idea was that, by giving the embryos an extra day to grow in culture, we would be able to tell which embryos were growing "faster," and thus transfer embryos that may have a better chance of implanting and creating a pregnancy.
This approach worked pretty well, with pregnancy rates of 20-40% reported by many centers. However, several problems remained. First is the fact that embryos do not implant in the uterus at the 2-8 cell stage; growth must continue for several days in the uterus before implantation is possible. Normally, the 2-8 cell embryo is in the fallopian tube traveling towards the uterus. Thus, we were transferring embryos into an environment (the uterus) that wasn’t where they belonged under normal circumstances.
Second, we generally had to transfer multiple embryos (3-4 typically) into the uterus to help ensure that at least one would continue to develop an implant, leading to triplet pregnancies or greater. These multiple pregnancies carry much greater risks of complication for both the mother and the babies. On the other hand, it is possible that NONE of the embryos were capable of continued development. In this case, although we felt we transferred good quality embryos, pregnancy did not occur.
Transfer of blastocyst stage embryos on day 5 post-retrieval answers many of the above concerns. The blastocyst stage embryo consists of over 50 cells, and is the stage at which the embryo normally is capable of implantation. The embryo normally exists within the uterus at that stage. In the past several years, many advances have been made in our understanding of what embryos need to grow in the laboratory. In the past, only 20% or so of embryos could be grown to blastocyst stage, but with current techniques 40-50% of fertilized eggs can be grown to the blastocyst stage in culture.