PGS or Preimplantation Genetic Screening for Chromosomal Abnormalities Provides a Pre-Pregnancy Plan
Preimplantation Genetic Screening, PGS, is a technique used in conjunction with IVF to increase the
PGS and PGD (Preimplantation Genetic Diagnosis) are frequently confused with each other. These terms are not interchangeable, but instead refer to two different services. We use PGS when looking for random abnormalities in chromosome number, while we use PGD when we are looking for a specific genetic defect carried by one or both members of a couple
- Women experiencing recurrent miscarriages
- Women with repeated implantation failure with IVF
- Couples in which the woman has advanced maternal age
Understanding the PGS Process
We all have 22 pairs of chromosomes (autosomes) and one pair of chromosomes (X and Y) that determines our sex. When the sperm fertilizes an egg, the mother will contribute one chromosome to the embryo’s pair, and the father will provide the complementary chromosome.
Problems occur when an embryo has extra or missing chromosomes (aneuploidy) or a part of one chromosome breaks off and reattaches in a different place (a “translocation”) on another chromosome.
When a couple decides to have PGS conducted, they first undergo a routine IVF procedure, with ovarian stimulation, egg retrieval, fertilization, and embryonic culture (growth) in the laboratory. But then, before their excellent quality embryos are frozen, a small sample of 6-8 cells is gently removed from each embryo on Day 5, 6, or 7 following the egg retrieval.
Although several different mechanisms can be used to biopsy the embryos, we believe that using a laser allows us to take the best quality sample with the least risk of damage to the embryo.
Following biopsy, the embryos are frozen, the samples are sent to a genetics laboratory, and we await the results – which usually takes about 7-10 days.
PGS testing allows us to determine which embryos have a normal number of chromosomes, and therefore an excellent chance to become a healthy baby, and which embryos have an abnormal number of chromosomes and are therefore typically unable to develop normally. This process has been shown to result in a significantly higher chance for pregnancy and delivery, as well as a reduced risk of miscarriage. In addition, as we know that the chance for pregnancy is higher for PGS tested embryos, we usually only transfer a single embryo – reducing if not eliminating the risk of multiple gestation.
PGS Can Identify the Cause of Recurrent Miscarriage
Our Texas Fertility Center physicians may employ PGS to evaluate embryos from couples who have experienced recurrent pregnancy loss. The most common reason for early (typically first trimester) pregnancy loss is genetic, and this is usually associated with the embryo having either too many or too few chromosomes, a condition referred to as aneuploidy.
Some conditions that result from aneuploidy include Down’s syndrome and Turner’s syndrome. These two abnormalities are the most common examples of aneuploidy, and typically produce abnormal pregnancies that usually spontaneously miscarry.
PGD follows the same process as PGS. Six to eight cells are biopsied from each embryo on day 5, 6, or 7, and the cells are sent to the genetics laboratory. The laboratory analyzes the chromosomes within each cell to determine which embryos have the specific genetic disease that we are looking for, which embryos will be asymptomatic “carriers” of the disease, and which embryos are normal. The couple can then decide which embryo to transfer and which embryos to keep frozen, thus minimizing (although not completely eliminating) the risk of their child having the genetic disease in question.
Genetic testing and embryo biopsy with PGS or PGD helps your fertility specialist identify the embryos most likely to result in a successful pregnancy. To inquire about the benefits of Preimplantation Genetic Testing and applications for your unique situation, please contact the care coordinators at Texas Fertility Center.