Normally the embryo will be transferred on the second or third day of development, when it has 4-8 cells. However, there exists the possibility of transferring the embryo on the fifth day, when the embryo has more than 100 cells and is called a blastocyst. This technique usually increases pregnancy rates per embryo, but there is a chance that some embryos do not develop into blastocysts.
This technique is recommended for patients with previous implantation problems, patients who can not risk a multiple gestation (only one blastocyst is transferred) or when the patient wants to reduce the number of embryos to be frozen, since only the embryos which do not become blastocysts are frozen.
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