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Preimplantation Genetic Diagnosis and Screening: PGD and PGS
If you’re exploring high-tech family building options such as in vitro fertilization (IVF), surrogacy, or egg, sperm or embryo donation, you can have your embryos examined with preimplantation genetic testing: either preimplantation genetic screening (PGS) or preimplantation genetic diagnosis (PGD). PGS and PGD do not diagnose diseases. The lab techniques screen embryos for single gene defects or chromosomal abnormalities (extra or missing chromosomes).
Preimplantation genetic diagnosis is the term used to describe testing for a specific genetic disease that an embryo is at risk of inheriting from the parents. "Most reproductive endocrinologists reserve the term PGD for couples that have a single gene mutation, and they're trying to make sure that their offspring won't carry that disease," says Houston fertility doctor Randall Dunn, M.D., with Fertility Specialists of Houston.
Preimplantation genetic screening is a newer term, and it refers to the screening of chromosomes for aneuploidy (abnormal number of chromosomes). "Preimplantation genetic screening is the primary term for what has really hit the reproductive world over the last few years, and that's screening embryos for the chromosomal aberrations, which basically means copy number changes," Dr. Dunn says. "And so that is the term that most people use today when they are speaking to a couple based on age [who have had] repeated IVF failures, repetitive miscarriages, or where they know some of the pregnancies were genetically abnormal."
PGD and PGS involve testing cells from embryos that are created via IVF. One or more cells are removed and carefully examined for genetic abnormalities. At this early stage of development, all embryonic cells have the same genetic makeup and potential for development, and they simply continue to divide without the removed cell. Geneticists and specialists examine and analyze the embryos. If a problem is found, the woman or couple can decide to transfer only the “good” embryos.
The risk of damaging embryos at this stage is small and studies have shown that PGS/PGD does not increase the risk of birth defects. It’s important to recognize, however, that screening and diagnosis are not 100 percent accurate. As a result, PGS/PGD is not considered an alternative to the two primary screening tests used during early pregnancy — amniocentesis and chorionic villus sampling. Most genetic counselors would also recommend an amniocentesis and chorionic villus sampling once you become pregnant.
"Testing very small samples of embryos is very different from testing someone's DNA from a blood or an amniotic fluid sample," explains Lauren Isley, Genetic Counselor with Genesis Genetics Institute. "Couples who are interested in PGD should discuss the accuracy of the testing with a genetic counselor specializing in this technology. Like PGD, PGS is also not perfect. The accuracy should be discussed in detail with couples interested in pursuing this test."
PGS/PGD costs from $2,500 to $7,000 and is not covered by most medical insurance policies.
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