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A blastocyst is a developmentally advanced embryo. Blastocysts have a defined structure. they appear as hollow spheres with a clump of cells on one side. Part of the cells that make up a blastocyst ultimately develop into the fetus. The remainder develop into the placenta and embryonic membranes (e.g. the amnion). Blastocysts will form roughly 5-6 days after egg retrieval. Since only a few embryos develop to this stage, each individual blastocyst embryo is thought to be more likely to implant. As a result, by waiting until the blastocyst stage (five or six days after the egg retrieval), even though fewer embryos are available, each embryo should have a better chance of implanting. This allows fewer embryos to be transferred.

Because each embryo has a higher likelihood of implanting, fewer embryos can be returned without decreasing the pregnancy rate. This will lead to a decrease in the risk of a high order multiple pregnancy.

Several things contribute to the likelihood of an embryo reaching the blastocyst stage. These include the chromosomal composition of the embryo (chromosomally abnormal embryos have a lower likelihood of reaching the blastocyst stage) and the ability of the embryo to adapt to the stress of the IVF process. For this reason, not every cycle will yield blastocysts. By monitoring the cycle closely for egg and embryo behavior, we are able to predict and prescribe which should proceed to a blastocyst embryo transfer.

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