Preimplantation Genetic Testing: Identifying Diseases, Preventing Miscarriages and More
Written By: Richard Hackett, MS on September 6, 2019
Preimplantation genetic testing began in the late 1980s to early 1990s simply by looking at sex chromosomes in embryos. In the years since, the complexities and techniques have vastly improved allowing us to learn more about embryos than ever before. Richard Hackett, Women & Infants Fertility Center lab supervisor, shares the current uses and processes of preimplantation genetic testing.
Performing genetic testing on IVF embryos
Genetic abnormalities can cause a variety of birth defects in children. But genetic abnormalities can also result in failure of an embryo to implant in a woman’s womb or the failure of a pregnancy once it is established (miscarriage). In fact, genetic abnormalities in embryos are a significant cause of infertility.
Genetic testing on embryos created through in vitro fertilization (IVF), known as preimplantation genetic testing (PGT), biopsies embryos to screen for such potential fertility failures, as well as to prevent birth defects in a resulting child. For the biopsy we take cells from the trophectoderm, which eventually become the placenta, to test them for a variety of abnormalities and potential problems.
We only remove cells from this outer area because the inner cell mass, the cells that eventually become the fetus, do not have any “extra” cells to spare. It is imperative that all those cells remain and divide. The trophectoderm cells give us an accurate characterization of the embryo without damaging it.
There are different types of PGT that we can perform on patients’ embryos.
PGT-A, or PGT for aneuploidy, is used to determine if embryos have the correct number of chromosomes (an incorrect number is called aneuploidy).
PGT-M, or PGT for monogenic disease, is used to identify an individual genetic disease.
PGT-SR analyzes the embryo for any structural rearrangements, such as a chromosome being in the wrong location.
We often use PGT when the patients have suffered recurrent miscarriages, meaning they have had two or more pregnancy losses. While we cannot always know why a miscarriage occurs, we do know genetics play a role. Other causes of miscarriage, such as structural abnormalities, would be detected with other testing procedures ordered by the physician. Women over the age of 35 have a higher chance of having a miscarriage. According to the American College of Obstetricians and Gynecologists, miscarriage occurs in more than one-third of pregnancies in women 40 years of age or older.
If we know one or both of the parents is a carrier for a known genetic disorder, we may recommend PGT. A family history of genetic disorders increases the chances of the embryo and subsequent child having that disorder.
PGT tests for more than 100 different disorders but some of the most common include cystic fibrosis, Huntington’s disease, Tay-Sachs disease and BRCA gene mutations. Using PGT, we can select embryos, if available, that do not have one of these genetic abnormalities.
If patients have multiple embryos from IVF, they may choose to select the gender of their embryos to transfer for pregnancy with PGT, which reveals the sex of the embryo. While some may be interested in family balancing, sex selection also is valuable in identifying sex-linked disorders.
Often if there is an X-linked disease, a male offspring may have the disease, whereas a female may only be a carrier and have no symptoms.
Typically found during genetic testing, mosaicism means that not all cells in the embryo are of perfect quality. PGT in the past was able to only identify genetically normal or abnormal embryos, resulting in the embryo as being classified as having the right number of chromosomes as suitable for transfer. Now we can identify mosaic embryos, which are those with varying proportions of normal and abnormal cells.
Depending on the degree of mosaicism, it may make no difference. We would consider an embryo with less than 40% mosaic cells to be a low-level mosaic, and have it as an option to transfer.
We would also choose the embryo with no mosaic cells first, but lower level mosaic embryos still offer a valuable chance to achieve pregnancy but with a lower pregnancy rate and higher miscarriage rate. Certain low level mosaic embryos would not be a good choice to transfer. This is due to the possibility of a live born baby with a genetic disorder.
Three things patients need to know about preimplantation genetic testing
While it has advanced since the 1980s, we can’t always know everything with preimplantation genetic testing. We screen for a specific genetic disease, such as cystic fibrosis, but don’t screen for all genetic diseases. Many patients don’t realize that receiving no answer is an option for their testing. Often this is due to the quality of the embryo. It doesn’t happen frequently, but patients should be aware their testing might not offer an answer.
Another thing we may learn from this testing is that none of the embryos are what is known as euploid, or normal. This is typically due to the age of the woman and her eggs. We can test 10 embryos and all 10 can be abnormal, which is incredibly disappointing for patients. They could always try again, but it is a possibility.
Finally, patients should know that there are risks associated with PGT. There is a very small chance the tests could be wrong, or the embryo could be damaged during the biopsy, but these are low risks.
Genetic testing overall increases the chances of getting and staying pregnant. It may a good option for you after discussion with your physician, particularly if you are a reproductively older patient.