There are two different kinds of PGD, either to eliminate parental mutations or to test for chromosomal number (aneuploidy screening). The later is used widely, but is usually more harmful than beneficial, for fascinating reasons, that I will explain later. The former, eliminating mutations of carriers of genetic disease is extremely effective and beneficial.
WHAT IS EMBRYO BIOPSY?
When an embryo reaches the third day of development, it normally has eight cells. One or two of these cells, called “blastomeres”, can be removed from the embryo with micromanipulation technique. The embryo is usually unharmed, and can go on to develop just as though this one cell were never removed. Of course it is not always true that there is no harm. The same can be done with the trophoblast of day 5 embryos. You can then subject those cells to genetic analysis, and know the chromosomal composition of the embryos, and if they carry a specific disease-producing mutation. Ironically it lowers the pregnancy rate. But it is a big money maker, and often is used inappropriately.