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Pre-Implantation Genetic Diagnosis, or PGD, is new diagnostic procedure in which embryos are tested for genetic or chromosomal abnormalities prior to placement within the uterus. While it can be performed on eggs, this is rarely done. PGD is most commonly used to screen embryos to see if their cells have too many or too few chromosomes. This is known as aneuploidy screening. PGD can also be used to test for a genetic illness known as PGD for single gene mutation screening.
By screening for commonly encountered embryonic chromosomal disorders, infertile couple may benefit from this test if the female partner is over the age of 35 or if they have had several failed IVF cycles. In addition, couples suffering from recurrent pregnancy loss may benefit from PGD.
Chromosomes are sub-cellular structures that house one’s genes. All of the information that is needed to lead to an individual’s development is contained on these chromosomes and each one of us has 23 pairs of them. During human reproduction, it is not unusual to produce embryos that have too few or too many chromosomes. This is known as aneuploidy (too few or too many chromosomes). Down syndrome is a form of aneuploidy in which the individual has 3 copies instead of 2 copies of chromosome 21.
As a women ages, she is more apt to mature eggs that contain an abnormal number of embryos. This is also seen in some case of recurrent pregnancy loss and has been observed amongst couples experiencing multiple IVF failures. Aneuploid embryos are less likely to implant than normal embryos. When they do implant, aneuploid embryos almost always miscarry. PGD was designed to screen for some of the more common chromosomal abnormalities in order to help select which embryos to transfer to the uterus in order to optimize the selection process.
PGD for aneuploidy screening does not offer information on birth defects and cannot detect a genetic illness. Screening for a genetic illness can be done by performing PGD for a single gene mutation. In this situation, the embryos are screened not for a chromosomal problem, but for a single gene abnormality. The tests require the same specimen, an embryonic cell, but are technically different and run in different labs.
