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A blastocyst is an embryo that has advanced to the five or 6-day stage of development. It is the stage of development that the embryo reaches when it implants in the uterus. The structure of the blastocyst is much more complex than that of a Day 2 or Day 3 embryo because it has many more cells and these cells are differentiated into 2 types: the trophoectoderm, which will later become the placenta and the inner cell mass which will give rise to the fetus. A blastocyst also has a central cavity full of liquid, which is called the blastocoel.
The blastocyst culture and embryo transfer on the Day 5 help us select the embryos with the best quality and implantation potential, significantly increasing the possibility of pregnancy. Less than 50% of Day 2 embryos reach the blastocyst stage. If there is a problem with the eggs or the sperm, at times no embryos attain the blastocyst stage.
The goal of Assisted Reproduction is to obtain a single healthy take-home baby. The best way to avoid twins is to transfer only one embryo. The most favourable moment to transfer a single embryo and achieve pregnancy is at the blastocyst stage. More and more specialists recommend transferring only one embryo, especially in women over 40 who use donor eggs. This is called SET or Single Embryo Transfer.