Genes carry instructions that control the development and function of the human body. Genes control all physical and medical characteristics, such as hair color, blood type and susceptibility to disease. A genetic disorder is a disease caused by a mutation in one gene, by mutations in multiple genes, by a combination of gene mutations and environmental factors, or by damage to chromosomes (the structures that carry genes).
Some diseases are inherited from parents and are present in an individual at birth, like sickle cell disease. Other diseases are caused by acquired mutations in a gene or group of genes that occur in the egg (or sperm). Such mutations are not inherited from a parent, but occur randomly. Thus, some genetic disorders show up spontaneously in one individual, with no previous family history. Descendents may inherit a disease or be carriers of the disease without being affected.
Three days after a woman’s egg is fertilized during IVF, a healthy embryo is usually at a 6 to 8 cell stage. For preimplantation genetic testing, one cell is removed from the embryo. This neither damages the embryo nor interferes with normal development. The embryo compensates for the removed cell. In the lab, specialists examine the cell for genetic diseases and disorders. If the cell passes the tests, the embryos are cleared for transfer to the woman’s uterus completing the IVF process.
Aneuploidy screening examines a specific set chromosomes to make sure they are normal. These chromosomes generally cause recurrent miscarriages. Aneuploidy screening may be done during preimplantation genetic diagnosis (PGD).
Aneuploidy screening requires harvesting of a woman’s eggs, fertilization, and sampling of embryonic cells. Using sophisticated new tools, cells from the embryo can be tested for the number of chromosomes. Ideally, there are 23 pairs of chromosomes, plus X and Y chromosomes if it’s a boy, or two X chromosomes if it’s a girl.
Aneuploidy is a term that describes a situation where there is only one of a certain chromosome instead of a pair (monosomy), or if there are three instead of two (trisomy). For instance, a common trisomy is when three number 21 chromosomes are present, which usually leads to pregnancy loss or a baby born with Down’s Syndrome.
If a specific single genetic disorder is known from the family history, PGD can be used to check for the disorder in every embryo. Only normal embryos are transferred to the woman’s uterus, reducing but not totally eliminating the chance of an abnormal embryo. However, risk of miscarriage is dramatically lowered after PGD because many pregnancy losses are caused by aneuploidy conditions.
Preimplantation genetic diagnosis (PGD) is not 100 percent reliable and does not completely replaced prenatal testing. However, two common methods of diagnostic prenatal testing – chorionic villus sampling (CVS) and amniocentesis are typically recommended later in a woman’s pregnancy, and cannot detect genetic disorders before pregnancy.