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Preimplantation Genetic Diagnosis
Couples at high risk of having a baby with a chromosomal disorder should know about a technology called preimplantation genetic diagnosis (PGD). This procedure allows doctors to check the chromosomes of an embryo before it implants in the uterus. Used in conjunction with in vitro fertilization (IVF) to screen for more than 200 chromosomal abnormalities, it’s recommended for couples with a high risk of passing on a genetic or sex-linked genetic disease.

During in vitro fertilization, a woman is given medication to stimulate the production of several eggs, which are removed and combined with sperm in a laboratory. The fertilized eggs mature for three days and develop into embryos. Before transferring embryos into the uterus, one or more cells are removed from each embryo for DNA analysis. While the remaining embryo grows, DNA analysis is performed using a technique called fluorescence in-situ hybridization (FISH). The extracted cells are tested with a fluorescent DNA probe that allows abnormal chromosomes to be detected with a special microscope. Following this evaluation, only the healthy embryos are transferred to the uterus.

Some of the chromosomal abnormalities that can be screened for are Duchenne muscular dystrophy, hemophilia A, Tay-Sachs disease, Fragile X syndrome, cystic fibrosis, Down syndrome and testicular feminization. PGD may be used by couples with a history of having a pregnancy with a chromosomal disorder, with a sex-linked genetic disease, with multiple miscarriages thought to be related to chromosomal problems, and by couples who have failed to become pregnant with traditional IVF.

Although it is expensive and invasive, PGD can provide tremendous reassurance to couples at high risk for having a child with a genetic abnormality. PGD has a 90 percent success rate. About 10 percent of the time there may be a false positive—the test finds a problem although the embryo has normal chromosomes—or a false negative—the test fails to find a problem that exists. Because the possibility for error in diagnosis exists, couples may have genetic testing later in the pregnancy using chorionic villus sampling (CVS) or amniocentesis.

Like any IVF procedure, PGD is accompanied by stress and often, disappointment. Fewer than half of IVF transfers result in live births. Couples need to balance the financial and emotional burden of the procedure with that of terminating the pregnancy or raising an affected child conceived naturally.

Any couple interested in this technique should contact an assisted reproductive technology centre to learn whether PGD is offered.