In usual treatments, the embryos are cultured for 2-3 days after the fertilization. The embryos can be divided into 4-8 cells before the transfer to the uterus. At this stage, the selection of the most vital embryos from all embryos of good quality may be difficult.

By culturing embryos for a longer period of time (the prolonged cultivation), we are able to choose the most vital ones for transfer, i.e. embryos that have continued to divide and develop. In extended culture, the embryos are cultured for 4-5 days prior to the transfer to the uterus. At this stage, the embryo already has more than one hundred cells, and it is at either the morula or blastocyst stage at the moment transfer.

This corresponds to the development stage of an embryo formed in natural conception as it passes through to the uterus. At the morula stage the embryo forms a cluster of cells where we no longer can count the exact number of cells. At the blastocyst stage the first differentiation of cells has occurred. The inner cells in the thickened area of the blastocyst develop into fetal tissue, and the surrounding cells, known as trophoblast, develop into the placenta at the fetal end. At the blastocyst stage the embryo is ready to attach onto the lining of the uterus once it has hatched out of its membrane.

In the past most embryos produced with IVF were transferred on day three of development, known as cleavage stage. When an embryo reaches five days of development it is called a blastocyst. Currently, with advances in understanding of the needs of developing embryos, the ability to produce blastocysts in the laboratory has increased. This extended culture time allows nature to help select those embryos with the highest capacity to produce a pregnancy. Culturing and transferring blastocysts on day five of development allows the transfer of fewer embryos while still maintaining a high pregnancy rate. Normally only two blastocyst stage embryos are transferred, thus reducing the risk of multiple pregnancies higher than twins.

Growing embryos in vitro to the blastocyst stage (day 5–6) for assisted reproduction offers several theoretical advantages over the transfer of cleavage stage (day 2–3) embryos. These include a higher implantation rate, a decrease in the number of embryos transferred, the opportunity to select more viable embryos for transfer, better synchronization between embryo and endometrium at the time of embryo transfer, and a longer time in culture that provides the opportunity to perform pre-implantation genetic diagnosis (PGD) when such is indicated.

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 embryo 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 may then be able to transfer fewer embryos and obtain an equivalent pregnancy rate with less risk for multiple pregnancy