Preimplantation Genetic Screening (PGS):

Why You Should Do It

Many factors can sabotage the success of infertility treatments, such as in vitro fertilization (IVF). A woman might respond poorly to ovarian stimulation, develop eggs that fail to fertilize, or produce embryos that are chromosomally abnormal. Her uterine lining might not be thick enough to support implantation, or there might be some other cause of failed implantation, such as damage to oocyte/embryo during unfreezing or transfer. The egg retrieval and embryo transfer procedure too can fail—sometimes due to the doctor’s lack of experience but more often due to uterine contractility. Of all of these factors, however, chromosomal abnormality is the one that controls the success of the majority of cases.

Fundamentally, the success of most infertility treatments relies on the simultaneous ovulation of multiple eggs. The hope is that at least one of those eggs will get fertilized, implant in the uterine lining, and develop into a healthy fetus. Statistically, if a woman is older than 35, a large percent of her eggs will be of poor quality and feature a chromosomal abnormality (e.g., a trisomy or monosomy). Such abnormalities almost always lead to a chemical pregnancy or a miscarriage later on in the pregnancy. How can they be detected and eliminated as a factor? By opting for an IVF with preimplantation genetic screening (PGS) of chromosomes.

Although PGS does not prevent chromosomal abnormalities from occurring, it increases one’s chance of achieving pregnancy as it identifies aneuploids (errors in cell division that result in a chromosome number that is not an exact multiple of the haploid number) and gives the doctor and the patient the opportunity to transfer only healthy embryos. With chromosomal abnormalities thus effectively eliminated from the list of possible contributors to a failed pregnancy, a woman then faces a roughly 85% chance of a successful pregnancy.

The way PGS works is by biopsying an egg or embryo and counting the chromosomes of a cell or cells. Normal cells contain 22 pairs of chromosomes in addition to XX (female) or XY (male), for a total of 46 chromosomes. A baby receives one of each pair from the mother’s egg and the father’s sperm. For some unknown reason, however, sometimes an egg or sperm will contain too many or too few chromosomes, jeopardizing the viability of the embryo. Even though such chromosomal abnormalities can be observed both with natural attempts to achieve pregnancy and with assisted reproduction methods, with IVF they can be detected before a pregnancy begins thanks to PGS, and the choice can be made not to implant these abnormal embryos.

To perform a PGS, an embryologist uses a small needle to collect a tiny sample of the egg or embryo for analysis. One of three methods can then be employed: (1) test the polar body from the outside of the egg, (2) test a blastomere (one cell) from an egg on the third day of development, or (3) test a trochoectoderm (multiple cells) from an embryo of the fifth day of development. After counting of the chromosomes, the egg/embryo is returned to the incubator (to continue growing) or is frozen for future use.

Note that PGS testing does not guarantee that a pregnancy will be achieved or that a pregnancy will result in a healthy baby free of genetic defects. One limitation of the screening is that it does not test for mitochondrial dysfunction or for the possibility of an inherited genetic disease. Testing for specific mutations is done via a different method called preimplantation genetic diagnosis (PGD) and is ordered only if, after genetic testing of the future parents has been performed, the couple have been determined to have a high risk of conceiving a baby with a genetic disease.

In our case, my partner was a carrier for a particular genetic disease, but I was not, which made our combined risk of having a child affected by the disease small. Thus, we opted to have only PGS done on all fertilized eggs that had grown sufficiently to allow for chromosomal screening. Had we not done PGS, my doctor would have selected embryos for transplantation based on embryo appearance, which is not always an accurate marker of the embryo’s health.

Our PGS results revealed that 8 out of 10 embryos had some chromosomal abnormality (monosomy 6, 7, 10, 15, or 22; trisomy 15, 16, 18, or 19; and a duplication on the short arm of chromosome 16), indicating exactly which 2 embryos had a chance of implanting and developing into healthy babies. You can now see the value of PGS: Had we not done it, we could have had potentially 8 failed pregnancies before having a successful one. Lots of time and money would have been wasted, and the emotional toll on us would have grown much heavier in what is already a highly stressful process.