What is Preimplantation genetic screening, PGS?
Preimplantation genetic screening is a technique used in conjunction with IVF where embryos can be screened for chromosomal abnormalities that arise during the development of the egg and embryo. Embryos are screened before they are implanted back into the uterus, so that abnormal embryos can be identified. Only embryos that can result in a normal child are transferred into the uterus.
What are chromosomal abnormalities?
All humans have 23 pairs of chromosomes in all of their cells. Chromosomes carry all the genetic material (DNA). Chromosomal abnormalities arise during the development of the egg or early stages of embryo formation. A chromosomal abnormality is when the embryo contains extra copies of chromosomes or not enough chromosomes. The chromosomal abnormality that is most familiar to people is Down Syndrome which occurs when the embryo contains an extra copy of chromosome 21 (trisomy 21). However there can be an abnormality (too few or too many) in any of the chromosomes. Most embryos with chromosomal abnormalities will not implant and therefore the patient would not get pregnant. However, chromosomal abnormalities are the most common cause of miscarriages. All women have embryos that contain chromosomal abnormalities, however as a woman gets older the chance that her embryos are chromosomally abnormal rises. There is a significantly higher chance of a chromosomally abnormal embryo in women over the age of 40. Chromosomal abnormalities are responsible for failure of implantation in a great extent of IVF cases.
How is PGS different from Preimplantation genetic diagnosis, PGD?
PGS is a technique where embryos can be screened for chromosomal abnormalities that arise during the development of the egg and embryo. These types of chromosomal abnormalities can happen in women of any age but are more likely to occur when women are older.
PGD is a technique where embryos can be screened for heritable genetic diseases or specific heritable chromosomal disorders. These types of genetic diseases are carried in the parents’ genetic material and are passed on to the genetic material in the embryo.
Who should undergo PGS?
PGS is useful in the following situations:
- Couples who have suffered several miscarriages
- Couples who have had several IVF failures
- Couples with repeated implantation failures
- Certain couples undergoing IVF for the first time
It is best to discuss with your fertility physician, if they believe PGS is right for you.
Should I see a genetic counselor before undergoing preimplantation genetic screening?
Patients who are considering PGS do not need to have a consultation with a genetic counselor to discuss their specific case. However, we do recommend genetic screening for heritable genetic diseases for all patients undergoing IVF. If a heritable disease is found in your genetic material then we have the ability to test for that disease in the embryos (PGD) in addition to the chromosomal screening. In this scenario, a counseling session with a genetic counselor is recommended.