Until recently, it was only possible to grow an embryo in the laboratory for three days. However, recent advancements have allowed embryos to grow from eight cell embryos (the third day) to multi-cell embryos (blastocysts) and transferred on the fifth day. This advance has benefited couples with multiple embryos available for transfer on the third day where it is historically been difficult to choose the best embryo(s). These advancements have led to a decrease in the multiple pregnancy rate in these select patients.

Overall, these patients achieve a higher pregnancy rate because transferring blastocysts is more likely to lead to pregnancy and transferring fewer blastocysts minimizes the chances of higher order multiples. However, there is a risk that few or no embryos will survive to the blastocyst stage.

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