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PGS: Who Is a Candidate for Preimplantation Genetic Screening?
Preimplantation genetic screening (PGS), also known as aneuploidy screening, is a screening test to look for structural abnormalities in chromosome number or size.
"Preimplantation genetic screening differs from PGD in both the technology that is used on the embryos, as well as what genetic material is examined in the embryo," says Lauren Isley, MS, Genetic Counselor at Genesis Genetics Institute. "PGS tests embryo samples for chromosome abnormalities, such as extra or missing chromosomes, which is collectively known as aneuploidy."
Most miscarriages can be attributed to aneuploidy (an abnormal number of chromosomes). An extra or missing chromosome is also a common cause of genetic disorders (birth defects) such as trisomy 21 (three copies of chromosome 21), which is found in Down Syndrome. The risk of aneuploidy increases as women age because the chromosomes in the egg are less likely to divide properly, leading to an extra or missing chromosome in the embryo.
"Common reasons that a couple may wish to pursue PGS testing would be a history of multiple miscarriages, a previous pregnancy or child with aneuploidy, advanced maternal age, or if one member of the couple is a carrier of a chromosomal translocation," Isley explains. "The majority of miscarriages are caused by chromosomal abnormalities in the developing fetus. Some individuals may be at an increased risk for conceiving a pregnancy with chromosome problems based on age or other risk factors."
With PGS, fertility doctors will transfer only chromosomally normal embryos into the uterus with the goal of reducing the risk of first or second trimester pregnancy loss. "Screening the embryo prior to implantation is thought to reduce the chance that an embryo with a chromosome abnormality would be implanted, and a miscarriage would ensue,"Isley says.
Doctors may recommend PGS for couples in whom:
- The woman is of advanced maternal age
- There have been two or more miscarriages
- There have been two or more unsuccessful IVF attempts (two separate cycles)
- The woman is over 35 and have had at least one miscarriage
- The male partner has severe male factor infertility.
Early studies of PGS on patients of advanced maternal age were promising and showed a decreased rate of miscarriages and a higher proportion of live births in the PGS group compared with a control group; however, a 2007 study published in the New England Journal of Medicine concluded that preimplantation genetic screening did not increase, and instead reduced the rates of ongoing pregnancies and live births after IVF in women of advanced maternal age. It is important to point out, however, that PGS was originally performed using a method of genetic analysis called fluorescence in situ hybridization (FISH), the method used in the 2007 study. Today, many labs are using more advanced methods of analysis called array comparative genomic hybridization (CGH) or single nucleotide polymorphism (SNP) analysis, which better screens the embryos.
Talk to your fertility doctor about whether PGS is something you should consider when you are doing in vitro fertilization (IVF).