embryo biopsy or preimplantation genetic diagnosis (PGD), diagnostic procedure, used in genetic screening, in which a single cell is removed from an embryo two or three days after it has been conceived through in vitro fertilization. At this age the embryo consists of about eight genetically identical cells. The embryo itself is unaffected and continues to grow while the selected cell's genes are replicated using polymerase chain reaction and then studied for genetic defects. The procedure allows an embryo to be tested before it is implanted into the womb when an inheritable disease or a genetic predisposition to a disease is carried by or exhibited in one or both parents.

Embryo biopsy has been used successfully to screen for such diseases as cystic fibrosis and Tay-Sachs as well as for genes that predispose a person to breast or colon cancer. The procedure can also be used to determine the gender of the embryo. Given this ability, X-chromosome-linked diseases that manifest only in males (hemophilia, for example) can be "screened" by implanting only female embryos (which will carry but will not develop the disease). Another variation, used for women over 35 (women past that age being statistically more likely to give birth to babies with Down syndrome and other chromosomal defects) involves examining the sample to make sure it has the correct number of chromosomes. Embryo biopsy has also been used to select a child who is a compatible donor for a sibling with a life-threatening disease such as certain anemias and leukemias.

See also amniocentesis; birth defects; chorionic villus sampling.