PGD is a method for knowing chromosomal and genetic abnormalities before implantation of the embryo.

The procedure is performed after the egg has been fertilized in vitro
and before the pre-embryo is transferred into the uterus.
After fertilization, the egg cleaves into individual cells called blastomeres.
In a normal development, all these cells should be virtually identical
to the genetic determination established during fertilization.
Therefore, studying only one blastomere, one can know the genetic
constitution of the embryo.
Due to near 40% of the eggs are arrested before 3rd day it is preferred to perform the biopsy when the embryo reaches the 8 cells stage on 3rd day in order to assure the study of live embryos.
PGD involves removing 1 or 2 blastomeres from the embryo with the help of  a micromanipultive microscope.  In this stage, all cells of the embryo are totipotential, this is why the retrieval of one or two of them is considered non damaging for posterior embryo development.
The biopsy may be also performed on day 5 with the advantage of the possibility of having more cells for the genetic study and of the blastocyst being a more viable stage of the pre-embryo during the in vitro development. The disadvantages are having less time for perform the genetic study and the cells removed correspond to trophoectoderm. The biopsed blastocyst may be vitrified and transferred in another better receptive cycle with the preparation of a more physiological endometrium.