“PGD performed on an embryo at the Blastocyst Stage(Day5) is safer than DNA sampling done on Cleavage Stage(Day3) embryos,” says Dr. Shefali Shastri, MD, FACOG
Pre-Genetic Diagnosis or PGD is a technique that enables people to look at the genetic make-up of an embryo prior to implantation. It can help individuals with a specific inheritance condition in their family to avoid passing it on to their children or by identifying aneuploidy (abnormal number of chromosomes). As you age, you are at an increased risk of genetic abnormalities. These abnormalities lessen your chance of getting pregnant and increase your risk for miscarriage.
There are several forms of PGD
RMANJ developed a unique, rapid, scientific method to improve the embryo selection process, called SelectCCS (comprehensive chromosome screening). Unlike FISH and other forms of PGD, SelectCCS has been rigorously validated before it was offered clinically. The SelectCCS validation process has demonstrated it is extremely accurate, that embryos diagnosed as abnormal virtually never result in healthy pregnancies and that using SelectCCS for embryo selection raises implantation rates to the point that single embryo transfer can be performed effectively across all age groups.
How does PGD work?
Following fertilization, embryos progress through multiple stages of differentiation. A day 3, or cleavage stage embryo, consists of eight cells called blastomeres, each of which have the potential to develop into placenta and or fetus. Day 5 embryos, or blastocysts, are made of approximately 200 cells and have clearly differentiated which cells are destined to be the placenta (trophectoderm) and which will become the fetus (inner cell mass).
Remember as I previously said, PGD involves removal of cells from a single embryo to assess the genetic make-up of that embryo. Biopsy of a day 3 embryo involves removal of 1 or 2 blastomeres from an eight cell embryo whereas day 5 trophectoderm biopsy involves removal of a few cells from the placental portion of a 200 cell embryo. Therefore, day 3 biopsies require removal of a greater proportion of the total embryo mass. Moreover, because each cell is less differentiated on day three, the cells that are removed from a day 3 embryo may have otherwise contributed to formation of the fetus. By contrast, day 5 trophectoderm biopsy removes a much smaller proportion of the overall embryo mass and those cells are exclusively from the placental compartment.
So, which biopsy technique is the safest??
Research recently performed at RMANJ and published in a peer reviewed journal (Fertility & Sterility) demonstrates that day 3 biopsy is harmful to embryos, resulting in a 39% reduction in the embryo’s chance to become a healthy baby. Obtaining a small amount of genetic DNA from the trophectoderm at the blastocyst stage (day 5) did not impair the embryo’s ability to implant. Furthermore, trophectoderm biopsy has been shown to be more accurate than cleavage stage biopsy because there is more DNA for genetic analysis.
Previous studies that have attempted to examine the potential benefits of PGD for aneuploidy screening on IVF pregnancy rates have failed to do so. This may be best explained by some of the limitations of the actual technique and timing of embryo biopsy. While the benefits of preventing genetic disorders may have outweighed this detriment, there now is a safer, more reliable option and trophectoderm biopsy is fast-becoming the standard of care.
For these reasons, RMANJ only performs biopsy at Day 5 when 200 or more cells have developed, and cells are taken from then trophectoderm or outer layer of the embryo.