Preimplantation Genetic Diagnosis (PGD)

Preimplantation genetics (PGD) is a state of the art procedure that is used to allows the genetic analysis of an embryo prior to transfer, thus prior to implantation and pregnancy. PGD must be done in conjunction with IVF.

Indications for PGD includes patients with history of genetic linked disorders, advanced maternal age, recurrent miscarriages, unexplained infertility, unsuccessful IVF cycles, or male factor infertility. The most common indication is a chromosome abnormality in one or both partners.

Some couples are at risk for transmitting genetic diseases to their children. These genetic abnormalities may occur in the sperm or egg and include aneuploidy (which are eggs or sperm with an abnormal number of chromosomes), structural abnormalities (translocations, deletions or inversions), or specific gene abnormalities (mutations). These genetic diseases include Tay-Sachs, hemophilia, sickle cell anemia, Thalassemia, Fragile X, Cystic Fibrosis, Marfans, Duchenne muscular dystrophy and many others.

Women over 35 are also at increased risk of miscarriage caused by aneuploidy. Preimplantation genetic diagnosis allows the physician and embryologist to screen embryos for many genetic diseases before they are transferred to the mother's uterus.

Couples choosing PGD must undergo an IVF cycle to produce embryos that can be genetically screened. IVF involves the administration of ovulation inducing drugs to the female, retrieval of the eggs once they are mature, exposure of the eggs to sperm so that fertilization can occur and culturing of the embryos in incubators.

Once the embryo matures, usually after three to five days of culture, a sample of its DNA is taken from one of the embryo's cell. A small hole is made in the membrane surrounding the embryo (the zona pellucida) using a solution known as Acid Tyrodes, a small needle, or a laser. This procedure does not damage the embryo.

Once the embryo's DNA is obtained it is examined using fluorescent in situ hybridization (FISH), to screen for abnormal numbers of chromosomes, and/or the polymerase chain reaction (PCR) to screen for single gene defects. FISH can also be used to identify a male and female embryo which is important since some genetic diseases are linked to the sex chromosomes. PCR is used to replicate certain areas of the chromosome so that they can be examined for the suspected disease.

If PGD indicates chromosomal abnormalities, the embryo will not be transferred back to the uterus. PGD can also be used to identify gender for couples seeking family balancing (but most clinics do not do this routinely).