What is Preimplantation Genetic Testing?
Preimplantation genetic testing (PGT) is a screening test used to determine if genetic or chromosomal disorders are present in embryos produced through in vitro fertilization (IVF). These tests were formerly named, preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS).
Why Should you Consider Preimplantation Genetic Testing?
Well over half of IVF treatments are unsuccessful because the embryo(s) transferred have the abnormal total number and/or distribution of chromosomes called aneuploidy. Ideally, we test embryos for abnormalities and then transfer a single embryo with the normal number and distribution of chromosomes called euploidy. PGT may be done with the intention of improving the chance of becoming pregnant, avoiding a miscarriage and having a healthy baby. The precision of selecting a single euploid embryo to transfer would serve to ease concerns related to multiple gestation (ex: twins), ovarian hyperstimulation syndrome early in pregnancy and preterm delivery with related complications.
Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are types of testing that can be performed on an embryo in the early stages of development.
What kinds of genetic abnormalities can PGT detect?
PGT assists in detecting several types genetic problems:
- PGT-A: Preimplantation Genetic Testing for Aneuploidy. This type of PGT assesses for the number and distribution of chromosomes within an embryo. A normal result is termed euploid. An abnormal result is termed aneuploid (without the normal number and distribution of chromosomes). Aneuploidy accounts for about 2/3 of the reasons an embryo fails to lead to a successful pregnancy or leads to an abnormal child if pregnancy advances to birth. One survivable example is Downs Syndrome caused by an extra chromosome “#21”. The vast majority of embryos with abnormal results do not survive to implant or result in a miscarriage within the first 12 weeks of pregnancy. Test results are estimated to be about 98% to 99% accurate. There is about a 1% to 2% chance the result is not correct.
- PGT-M: Preimplantation Genetic Testing for a Single Gene Mutation. This type of PGT is considered if a known single genetic abnormality is found in a family member and is carried in one or both parents posing a risk of having a child affected by this single gene abnormality. A chromosome contains many genes. In this situation, the parents provide cell samples from swabbing the inside of their mouth. The cells are tested to learn if the parent carries the abnormal gene. A marker probe for their specific abnormal gene is created and used to test an embryo cell for the presence of the same abnormal gene.
- PGT-SR: Preimplantation Genetic Testing for Structural Rearrangements.
Preconception Routine Genetic Carrier Screening is an Essential Step to Learn if You Should Consider PGT-M
PGT-M is most often used by couples struggling with issues related to a known family or personal history of a specific abnormality in a single gene contained within the much larger chromosome of every cell in the body. In 2001, the American College of Obsteterics & Gynecology and American Academy of Medical Genetics set forth guidelines recommending genetic testing for common gene disorders that behave recessively- a person can be a silent carrier of the mutation-not having the clinical disease caused by the mutation. For example, once the genetic disease is discovered in an affected family member, such as a first born son with Fragile-X syndrome, the parents will seek medical care to prevent having a second child with the same genetic disorder that leads to a lifetime of severe physical and cognitive disability. Genetic mutations cause diseases such as the ethnicity related group of diseases within the Ashkenazi Jewish members of our community, the hemoglobin abnormalities sometimes found in people with ancestry from Africa, the Mediterranean Sea and Pacific Ocean shores of the world or those independent of ethnicity, such as cystic fibrosis, spinal muscular atrophy (SMA) and fragile-X. Children born with these genetic diseases may fail to survive beyond birth or the first few years of life or could suffer greatly into adolescence and adulthood. If both parents are silent carriers, there children are a risk for having the disease. PGT-M helps to avoid having a child with these genetic diseases.
Testing both parents to learn if either or both are carriers (unaffected or mildly affected by the mutation) of the genetic mutation can be performed prior to trying to conceive. Genetic counseling is a key step in considering PGT-M and setting realistic expectations for benefits and risks. If there is significant risk of having an affected child or for miscarriage if an embryo is affected by the mutation, then the couple can undergo in vitro fertilization (IVF) therapy. Embryos that grow to a blastocyst can undergo a biopsy. Some cells are removed from the embryo without harming it. The cells undergo genetic testing. If the test results are favorable, then an embryo is suitable for transfer into the uterus.
Be Cautious About the Marketing Hype of PGT
Although the PGT results may be normal, the normal results do not absolutely ensure having a normal baby. There are many other causes for birth defects, miscarriages and embryos not surviving beyond the number and distribution of chromosomes or exclusion of testable genetic mutations. The test results themselves may fail to detect a genetic abnormality. PGT does not replace other genetic testing tools that can be used during your pregnancy called prenatal genetic testing. Your obstetrician will counsel you regarding the need for such testing.
There are websites that tout very high success rates with PGT, but the devil is in the details. Here’s an example of common marketing tactics. The webpage may claim a 70% ‘success’ rate for IVF with PGS in brightly colored graph form. In fine print, there is a description that the data is limited to women who had an embryo transferred. At first glance, it looks like this clinic has great success using IVF with PGT. In reality, 100 couples may have had IVF with intention for PGT, and 50 couples had PGT results with at least one normal embryo. For the remaining 50 couples, none of the embryos had a normal PGT result or they had no embryo with advanced development upon which to perform PGT. Therefore 50 couples did not have an embryo to transfer. Among the 50 couples with at least one PGT normal embryo, 35 become pregnant. The clinic displays a graph showing 70% ( 35 of 50) success for couples undergoing PGT (with an embryo transfer written in fine print). The 50 couples who had no normal embryo and no transfer were not included in the graph. It turns out that for all couples intending to have a baby with PGT to assist, only 35% (35 of 100) became pregnant.
What if PGT Doesn’t Result in a Pregnancy?
There is good hope to have a healthy child with preimplantation genetic testing. Even so, it’s important to know that planning to have PGT is no guarantee the embryos can be tested or, if tested, that there will be a normal result. If the PGT process brings bad news—such as embryos that don’t grow or results that show all embryos are genetically abnormal, at least there is discovery as to why IVF was not successful.
When things don’t work as hoped, the sense of loss and grief can be intense. Persistence is a powerful attribute to have and apply in this situation. Our Michigan Reproductive Medicine (MRM) team is here to support you every step of the way, cheering your success and supporting you through challenging and frustrating times. Our infertility support group led by former patients of our practice provides an amazing, organically patient-centered experience to all that participate.
Questions About PGT? Call MRM and Come In for a Consultation!
If you are considering in vitro fertilization and PGT and want to know if this treatment may be right for you, meet with one of our physicians at MRM for a consultation.
Call us at (248) 593-6990 or click here to contact us. You are also more than welcome to come visit our fertility center so you can see our facilities for yourself!
