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Bust a Myth about PGD/PGS
I'm having in vitro fertilization (IVF) — why not just go ahead and have the embryos genetically screened to make sure there are no chromosomal abnormalities. It’s really no big deal.
Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are genetic testing procedures for embryos. The procedures offer couples that are likely to pass on a genetic abnormality a way to become biological parents with minimal risk.
PGS screens embryos for chromosomal abnormalities to make sure they have the right number of chromosomes without any extra or missing. PGD screens for single-gene defects that may cause genetic disorders such as Cystic Fibrosis.
PGD and PGS require one extra step in the IVF treatment process. Once the eggs have been fertilized and embryonic development begins, the embryologist performs an embryo biopsy, and genetic specialists determine which embryos carry a genetic anomaly and which are normal.
The risks of PGD include damage to the embryo during the biopsy procedure, according to Serena H. Chen, M.D., a New Jersey reproductive endocrinologist with IRMS Reproductive Medicine at Saint Barnabas. "The good news is that embryos damaged by PGD appear to experience an "all or none" effect — they stop growing, rather than sustain long-term damage," she says. Embryos that continue to grow after the biopsy do not become abnormal as a result of the biopsy and are not at greater risk for miscarriage or birth defects.
However, “There is a general misconception that the whole embryo biopsy process is just another 'option' in the IVF/embryo transfer menu, like ICSI, assisted hatching, blastocyst transfer, cryopreservation and so forth. The most important thing to remember about PGD/PGS is that it is an invasive technology,” says Michael Tucker, Ph.D., Scientific Director and Chief Embryologist at Georgia Reproductive Specialists in Atlanta. “At least one cell or more must be removed from each embryo to enable the assessment to take place, and as such it is not a procedure that should be undertaken lightly. For single gene defects and chromosomal structural anomalies that have a high probability of appearing in the next generation, then such an invasive procedure as embryo biopsy is justifiable, even though the embryo viability might be partially compromised.”
Dr. Tucker says the benefits become much less clear when the biopsy is used for aneuploidy screening to minimize the chance that a transferred embryo has a chromosome abnormality linked to advancing reproductive years. “In such a patient population, the number of potentially viable, good quality embryos might be limited in any event,” he explains. “Embryo biopsy to screen for chromosomal errors as a means to do anything other than reduce the already low risk of conceiving an abnormal child, or to minimize the risk of miscarriage, is much less easily justified."
According to Laurence A. Jacobs, M.D., a reproductive endocrinologist with Fertility Centers of Illinois, the most common situations for recommending PGS include:
- Women age 39 or older (although some women 35-38 ask for the procedure)
- Severe male factor infertility (especially when testicular biopsy is needed to obtain sperm)
- Miscarriages (two or more genetic or unexplained losses)
- IVF failures ( two or more failures, despite normal appearing quality embryos)
Dr. Jacobs says the most common indications for recommending single gene PGD include:
- Previous birth of a child with a single gene disorder, such as Cystic Fibrosis, Tay Sachs, Muscular Dystrophy, Hemophilia, Thalassemia, fragile X or Sickle cell.
- Both partners are "carriers" for a single gene disorder based on screening tests and, therefore, at risk for passing on inherited genetic disease to their offspring.
When considering PGD/PGS, do your research and weigh the decision carefully. "PGD/PGS technology is changing rapidly, and the experience of different centers varies widely," says Dr. Chen. "So the decision about whether or not you should proceed is a very individual one based upon the risks and benefits in your individual situation. Your doctor should be able to assess these risks and benefits for you from her or his center's experience, so that you can make an informed decision."