A blastocyst is an embryo that has developed for five to seven days after fertilization. At this point the embryo has two different cell types and a central cavity. It has just started to differentiate. The surface cells, called the trophectoderm, will become the placenta, and the inner cells, called the inner cell mass, will become the fetus. A healthy blastocyst should begin hatching from its outer shell, called the zona pellucida by the end of the sixth day. Within about 24 hours after hatching, it should begin to implant into the lining of the mother's uterus.

The ultimate goal of in vitro fertilization (IVF) and embryo culture is to provide high quality embryos which are capable of continued normal development and result in live births. However, under standard IVF culture conditions, only about 20-40% of human embryos will progress to the blastocyst stage after 5 days of culture. This low rate of embryo development is partly due to the result of a less than optimal culture environment for the embryos. For this reason, embryos have usually been transferred into the uterus after only 2-3 days of culture.

One problem with this is that 2 to 3-day-old embryos are normally found in the fallopian tubes, not in the uterus. The embryo first moves into the uterus at about 80 hours after ovulation. The implantation process begins about 3 days later - after blastocyst formation and hatching have occurred. Therefore, if in vitro culture conditions could be improved so that blastocysts formed at a higher rate, then embryos could be placed into the uterus at the blastocyst stage - at a more "natural" time, and shortly before implantation should occur.

Transferring blastocysts following IVF also provides another benefit - reduction of the possibility of multiple pregnancy. Some 2 or 3-day-old embryos do not have the capacity to become high quality blastocysts and a viable pregnancy. However, on day two or three of culture we do not have reliable methods to determine which embryos will be viable long-term. By culturing embryos to the blastocyst stage we have more opportunity to choose the most competent ones for transfer. We can then transfer fewer embryos and obtain high pregnancy rates with less risk for high order (triplets or higher) multiple pregnancy.

In the past, it was very difficult to get good numbers of high quality blastocysts with in vitro culture systems - unless "feeder" cells were utilized - coculture. However, new culture media have recently become available that yield much higher blastocyst formation rates. This makes blastocyst transfer a viable option for some couples with infertility.

With the success our clinic has with day 3 transfers, only a few select patients have been selected for blastocyst culture and transfer.