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A blastocyst is an embryo that has developed in culture for at least five days after fertilization and has divided into two different cell types. The surface cells are termed the trophectoderm and will eventually become the placenta and the inner cell mass will become the fetus. A healthy blastocyst should hatch from its shell (zona pellucida) by the end of six days or earlier and begin to implant within the lining of the uterus.
In the early years of human IVF, embryos were transferred to the uterus between 24-48 hours (Day-1 or Day-2 transfer) after fertilization.
This was a more physiological approach since it is known that
under in-vivo conditions the embryos reach the uterine cavity around the end of the third day following fertilization. Three day old embryos have usually developed to the 6- to 8-cell stage.
Since we cannot determine the viability of each embryo at this stage, and given the high embryonic loss at this stage, it was common to replace many embryos to compensate for those that would not implant.
This practice yields adequate pregnancy rates, however, it often resulted in high order pregnancies. Usually, two blastocyst stage embryos are transferred, compared to four or five day-3 embryos. This dramatically reduces the probability of multiple pregnancies with higher pregnancy rates in many cases.
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