Screening embryos to see if they are healthy By on November 06, 2012

Our Los Angeles based fertility practice, the Center for Fertility and Gynecology, is experienced and adept at all modern forms of fertility technologies. One of the newer technologies which is gaining popularity is called preimplantation genetic screening or PGS for short. PGS is a method of checking the chromosomal content of an embryo before it is placed into the womb.

PGS and PGD

Preimplantation genetic screening originally was developed as preimplantation genetic diagnosis, or PGD. PGD is used when there is a specific disorder that the embryo is at risk for developing. These circumstances often include hereditary diseases such as Cystic Fibrosis and Tay Sachs disease.

The Process

In either case, PGS or PGD, the steps are as follows. First, as in standard IVF, medication which stimulates multiple egg growth is given to the mother. After approximately 9 days of medication, the eggs are removed from the ovaries through a short procedure under light anesthesia. Once the eggs are obtained, the father’s sperm is added to the eggs to facilitate fertilization. Often this step is enhanced through the use of ICSI (intracytoplasmic sperm injection). The day after the egg and sperm are mixed, the embryo usually contains 2 pro-nuclei (2PN), the first sign that fertilization has occurred. Subsequently, the embryo divides into 4, then 8 cells by day 3. At this point, one of the eight cells can be gently removed from the embryo. That removed cell can then be examined to determine if its chromosomes are normal or abnormal. For example, it can be determined if the gene for a specific disease like Cystic Fibrosis is present or not. Once this is determined, the embryos which contain the normal chromosomes can be placed into the womb, the embryos with abnormal chromosomes are not.

Day 5 biopsy

For PGS, the process is slightly different simply because the biopsy usually occurs on day 5 after fertilization as opposed to day 3. When the biopsy is done at day 5, more than 1 cell is obtained. This provides more information about the embryo and allows more accurate conclusions to be drawn about all of the chromosomes (as opposed to a single gene defect). This day 5 biopsy is called a trophectoderm biopsy and refers to the specific type of cells which are removed from the embryo. Once the cells are removed all of the chromosomes are examined to be sure that there are a correct number of copies for each. If there are too many of any one chromosome (such as 3 copies of chromosome 21 which causes Downs syndrome) or too few copies of a chromosome (such as 1 copy of chromosome X which causes Turners syndrome) the embryo is not placed back into the womb.