PGD is a technology that allows testing of early embryos for the presence or absence of a genetic disorder, even before the embryo is transferred into a mother’s uterus.
Couples who are at increased risk for transmitting a known genetic disease to their children now have an option apart from traditional prenatal screening during the pregnancy. PGD can be used to evaluate sex-linked genetic disorders such as hemophilia, specific types of muscular dystrophy, other severe genetic diseases that involve a single abnormal gene (cystic fibrosis, Tay Sachs), and abnormalities due to an abnormal number of chromosomes (Down’s Syndrome, other trisomies). PGD may be an option for families that would otherwise be forced to make difficult decisions when a pregnancy is already well-established. PGD is specifically applicable for couples who are carriers of defective genes that may cause a specific genetic disease in their offspring.
Examples of these genetic diseases include Cystic Fibrosis, Fragile X Syndrome, Sickle Cell Anemia, Tay Sachs, Hemophilia, Thalassemia, and many more. PGD has also been applied when one spouse has a chromosomal defect known as a balanced translocation that greatly increases the risk of miscarriage. Very specific molecular biology tests must be developed directly for each disease. As time progresses, more diseases are added to the list of those that can be diagnosed using PGD.
PGD’s indications also include aneuploidy screening. Normally, there are two copies of each chromosome in each cell. Aneuploidy refers to a genetic abnormality in which there are either too many or too few chromosomes present. Infertility research is continuing to evaluate whether basic aneuploidy screening may increase IVF success rates and possibly decrease miscarriage rates and newer studies are very promising.
PGD occurs in conjunction with an in vitro fertilization (IVF) cycle. Eggs are fertilized in the laboratory, and the resulting embryos are cultured for several days. During the several day culture, a small opening is made in the embryonic cell membrane of each embryo and a single cell is withdrawn. During this early embryologic stage, each cell is essentially a stem cell, and the remaining embryonic cells can regenerate. PGD biopsied cells are then used for specific testing.
Embryo Selection and Transfer
The embryo transfer procedure is very similar to an IUI. While watching with an ultrasound, a small catheter is introduced through the cervix and into the uterine cavity. An even smaller catheter is then loaded with the embryo(s) and passed through the first catheter. The embryo(s) is then pushed out into the uterus where it will hopefully implant. After the transfer, patients are asked to rest in a lying position for approximately 30 minutes.