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 In vitro fertilization (IVF) involves several steps including combining the sperm with the eggs and incubating the resultant embryos. The embryos are kept in the incubators until ready for transfer, which is usually 3 days until the 6-8 cell stage.
A blastocyst is an embryo that has been cultured 5-6 days and has differentiated into two distinct cell types know as the trophectoderm, which will become the placenta and the inner cells that will become the fetus. In most cases, blastocysts are “more hearty” than day 3 embryos. As the embryos develop past three days, those that are weaker do not survive and others from blastocysts.
Because blastocysts are more likely to implant and survive, fewer can be transferred to the uterus those lowering the incidence of multiple births. High order multiple births (>3) can increase the mothers health risk and greatly increase health care costs.
It is not always possible, or desirable, to culture embryos to the blastocyst stage. There have to be enough embryos available to “risk” loosing those that will not culture to blastocysts. For example, if there are only 2-3 embryos, none may mature to blastocysts and the cycle would be “lost”.
When there are enough embryos, and other patient specific factors are favorable, transferring blastocysts may increase pregnancy rates while lowering the incidence of multiple births.
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