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Transferring embryos at the blastocyst stage also provides a better co-ordination between the embryo and the uterus by putting the embryo back in the right place (the uterus) at the right time (blastocyst stage).
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A frozen embryo transfer, or FET, is a kind of IVF treatment where a cryopreserved embryo created in a full IVF cycle is thawed and transferred to a woman's uterus.