PGT-A or PGT-SR is used to determine whether the cells in an embryo contain the normal number of chromosomes, which is 46. After an embryo grows in the lab, it is usually biopsied on day 5 (blastocyst stage). A few embryo cells are then sent to an external lab which uses technology to count the number of chromosomes within each cell. Embryos with a normal number of chromosomes are “euploid” and those with an abnormal number are “aneuploid.” The purpose of PGS is to avoid transferring an abnormal embryo into the uterus.
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