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Limitations of PGD and PGS


What Genetic Testing Can and Can’t Do

Pre-implantation genetic diagnosis (PGD) and pre-implantation genetic screening (PGS) are genetic testing that can be performed on embryos before they are transferred to a woman’s uterus as part of IVF treatment. This testing can screen for some types of genetic diseases and disorders, so embryos without those defects can be transferred.

PGD or PGS may be especially important to you if you or your partner has a family history of genetic diseases, if you have had several miscarriages, or if the female partner is older, since older eggs are more likely to have chromosomal disorders. Either test may be used for gender selection, which some people are interested in for family balancing.

Each of the tests has its purpose, and each has limitations. Neither one can absolutely guarantee that you will have a healthy baby. Your reproductive endocrinologist will advise you if you should consider PGD or PGS, and may also recommend genetic counseling. Let’s look at what each test can and can’t determine.

PGS: Testing for Chromosomal Disorders

Chromosomes contain all of your DNA. Humans have 46 chromosomes in 23 pairs. During fertilization the embryo normally receives 23 chromosomes from the egg and 23 chromosomes from the sperm. Chromosomes 1 through 22 carry the genetic material, while 23 determines the sex of the embryo.

PGS tests for a normal number of chromosomes. It provides screening for disorders such as Down syndrome or Turner syndrome which are caused by an abnormal number of chromosomes in certain positions. Chromosome disorders typically are not inherited and are more likely to happen when parents are older, especially the mother. PGS also determines the sex of the embryo. Some fertility clinics offer PGS for non-medical purposes since it allows parents to choose the sex of the implanted embryos.
PGS does not screen for specific genetic disorders which are inheritable, such as Tay-Sachs disease or cystic fibrosis. False positives and false negatives are possible, although the risk is less than one percent. The test isn’t able to study every chromosome, so it cannot guarantee you will have a healthy baby. This is why your obstetrician may recommend prenatal testing a bit later in the pregnancy to screen for chromosomal abnormalities.

PGD: Screening for Specific Disorders

PGD identifies mutations which are caused by recessive genes and result in inheritable genetic disorders, including Tay-Sachs disease, cystic fibrosis, sickle cell anemia, muscular dystrophy, Gaucher’s disease, and others. These mutations happen when both parents are carriers of the recessive gene and the embryo receives a recessive gene from each of them. PGD can test for over 100 different genetic conditions, including sex-linked disorders (disorders carried only on the X chromosome or the Y chromosome).

If there is a history of genetic disease in your family or your partner’s family your fertility specialist may recommend PGD. PGD also determines the sex of the embryo and may be used for gender selection as well.

As with PGS, false positives and false negatives are possible, although the risk is less than one percent. PGD helps reduce the risk of carrying a child with genetic disease, but it can’t completely eliminate the risk. Prenatal testing is recommended later in the pregnancy to screen for genetic disorders. Another consideration is that some genetic diseases don’t have symptoms until the carriers reach middle age. That is a tricky ethical problem to confront. If the genetic disease which runs in your family is of that type, discuss the implications with your physician.

Do You Need Genetic Testing?

If you or your partner has a family history of genetic disorders or you are an older woman, you may want to consider genetic testing as part of your IVF treatment. Most fertility centers offer this type of testing for medical purposes. Not every fertility center will perform the tests for gender selection only.