PGD Biopsy:
Three types of biopsies are possible: Polar Bodies, Blastomeres and trophoectoderm cells.

Polar bodies biopsies: As the first polar body biopsy allows to infer the outcome of the first maternal meiotic division and errors may also occur during the second division it is also necessary to test the second polar body in order to avoid a misdiagnosis. The second division of the egg is completed when the sperm penetrates and fertilizes the egg. Therefore, the 2nd PB biopsy is performed once the egg has been fertilized.
The testing of the biopsied polar bodies allows to infer only maternal inherited problems, therefore they are not suitable for paternally inherited disorders. Considering this inconvenient it is preferable to perform the blastomeric biopsy, which allows evaluation of the maternal and paternal contributions to the embryo and the errors subsequent to first cleavage.

Blastomere biopsy: In contrast, blastomere biopsy can test maternally and paternally derived conditions, as well as X-linked disorders by gender determination.
For this reason and because not all eggs originated will survive the first stages of the pre-embryo development ,the blastomere testing has become the most popular and accepted method of P.G.D.

Trophoectoderm biopsy: Performed at the blastocyst stage on day 5 of the pre-embryo development. Usually the zona pellucida is perforated on day 4 and the protruded cells of the trophoectoderm are aspirated on day 5. The advantage over blastomere biopsy is that several cells are aspirated instead of one or two, leading to an easier genetic study, and besides, it is performed in a pre-embryo with more potential for implantation. The disadvantage is that the study corresponds to the trophoectoderm and should be completed in less to 24 hs when the transfer is planned to be done in the superstimulated cycle; otherwise, the embryo would have to be cryopreserved for future transfer.