Genetic selection procedures are done either on fetuses, through prenatal screening, or on embryos that are outside a woman’s body, through Preimplantation Genetic Diagnosis (PGD).
PGD tests embryos for the presence of genetic sequences linked to a variety of conditions and characteristics. A cell is extracted from an embryo at its eight-cell stage and analyzed. Embryos with the selected characteristics can be implanted in a woman's uterus to develop into a child. The procedure does not appear to affect embryos’ or fetuses’ subsequent development, though more follow-up studies of children born after PGD are needed.
PGD was developed to allow couples at risk of passing on a serious genetic disease to have children not affected by it. Since its introduction in 1990, it has been most widely used to prevent the birth of children with conditions such as Down's syndrome, Tay-Sachs disease, cystic fibrosis, sickle cell, Huntington's chorea, and Cooley's anemia.
However, PGD is increasingly being used for other reasons. These include social sex selection, creating “savior siblings” who can provide bone marrow or other transplant tissues to sick older siblings, and selecting against embryos with genes correlated with late-onset and non-fatal conditions. Some clinics have even offered the technique for purely cosmetic traits including eye color, hair color, and skin complexion.
A newer variation of PGD, called Preimplantation Genetic Haplotyping, allows for many more genes to be tested, and for greater accuracy.
Many disability rights advocates, in particular, have been critical of PGD and prenatal screening. They point out that the definition of "disease" is to some extent subjective. Most support women’s right to decide whether or not to have a child at a given time, but are critical of basing this decision on the traits of the particular embryo or fetus.