Preimplantation genetic screening, or PGS, involves testing whether an embryo has the correct number of chromosomes before transferring it back to a woman’s uterus. This allows us to help determine which embryos have the best chance of becoming a healthy baby.
Five or six days after egg retrieval, at the blastocyst stage, a few cells are removed from the embryo. The cells are removed from the area of the embryo called the trophectoderm; these cells are destined to become the placenta, not the fetus. By avoiding transferring genetically abnormal embryos, this procedure reduces the chance of miscarriage, or when done to identify specific genetic disorders, may prevent the need for termination of pregnancy or the birth of a baby with a serious genetic disease. When genetic testing is performed to determine whether an embryo carries a specific disease-causing mutation it is called Preimplantation Genetic Diagnosis (PGD). Reproductive Partners Fertility Center – San Diego (RPSD) delivered the first PGD baby in San Diego County in 2001.
Improvements in Technique
The techniques for PGS have improved greatly over the past several years. Studies have found that biopsy of the embryo at the blastocyst stage does not negatively impact the embryo’s ability to implant into the uterus. In previous years when embryo biopsy was performed on the third day after fertilization there did appear to be a negative impact on the embryo’s ability to implant, and biopsy at this earlier embryo stage was more prone to error. Furthermore, technology now allows us to perform comprehensive chromosomal screening (checking all the chromosomes). Thus, our current method of blastocyst biopsy is highly accurate and does not appear to pose significant risk to the embryo.By avoiding futile transfers where the embryo is destined to fail to implant or to miscarry, the birth rate per transfer is markedly increased.
PGS is often recommended for women over 35 years old. As maternal age advances, a higher proportion of embryos are genetically abnormal. This higher prevalence of genetically abnormal embryos is the primary contributor to the decreased chance of successful pregnancy and increased chance of miscarriage in older women.
PGS may be considered when both partners are carriers for a specific genetic disorder, such as Cystic Fibrosis (CF). With CF, PGD is performed because one fourth of the embryos would carry both abnormal genes and be affected with this serious disease. With some other genetic diseases, half of the embryos will be abnormal. Also, patients may have an increased risk for miscarriage because one of the partners carries a genetic rearrangement in their chromosomes (translocation carrier), and genetic testing of embryos for these couples may reduce the risk of miscarriage.
Elective Single Embryo Transfer
PGS can also help us electively transfer a single genetically normal embryo, even for older women, and still offer couples an outstanding chance of success. By transferring only a single embryo, we can help avoid a twin pregnancy, which has much higher risks of complications for both mothers and babies. Studies have shown that electively transferring a single genetically normal embryo can increase the chance of having a healthy baby.