Preimplantation genetic testing (PGT) can screen embryos for chromosome abnormalities before they are put in the uterus. This can help to reduce the odds of a genetic problem in the baby that might otherwise lead to a miscarriage or a failed IVF cycle. PGT is different from prenatal screening (e.g. NIPT, FTS, TRIO, SIPS) because those tests examine an existing pregnancy, usually after 10 weeks gestation.
PGT-M and PGT-A is the modern terminology, but in the past PGT was referred to as ‘preimplantation genetic diagnosis (PGD)’ or ‘preimplantation genetic screening (PGS)’ or ‘comprehensive chromosome screening (CCS)’. The IVF industry has standardized the terminology now and we use the term PGT.
Preimplantation genetic testing for monogenic/single gene defects= PGT-M (e.g. Huntington’s disease, Muscular Dystrophy, BRCA breast cancer gene)
Preimplantation genetic testing for aneuploidy = PGT-A (e.g. testing for missing/extra chromosomes that would lead to a non-viable embryo or extra chromosomes that cause genetic differences like Trisomy 21 – Down Syndrome)
PGT-M and PGT-A help examine an embryo, even before a pregnancy begins. PGT starts with the IVF process, where the egg is fertilized with sperm, in the embryology lab. Five or six days after fertilization, a biopsy is performed, where a small sample of cells are removed from each embryo, frozen, and sent off to a genetics lab. These cells are tested for the condition of interest and the embryos predicted to be unaffected are transferred to the woman’s uterus the following month, or later.
The biopsy of the embryos for either PGT-M or PGT-A is performed at PCRM by our skilled embryology team. The cells are sent to a specialized laboratory, typically Cooper Genomics, for analysis. After the biopsy, embryos are vitrified (frozen) and stored until the genetic testing is completed. PGT results are ready about 1-2 weeks after the biopsy.
In a standard IVF cycle (.ie. without PGT), embryos are graded and selected for use by their appearance (morphology). When an IVF cycle includes PGT, the selection of embryos is based on their genetic makeup. Embryos predicted to be unaffected can be transferred back to the uterus at any time, even as soon as the next cycle.
Couples may choose PGT for a number of reasons:
- They wish to screen embryos before transferring them, in the hopes of choosing the one with the highest chance of success
- They have undergone standard IVF cycles that have been unsuccessful or resulted in miscarriages.
- One partner carries an inherited chromosome rearrangement, such as a translocation or inversion.
- They have a child with a genetic condition and would like to reduce the chance of inheritance for their future children.
- An adult has a genetic condition or has an increased risk of developing a genetic condition and would like to reduce the chance of inheritance for his or her future children.
The potential benefit of these techniques is determined during the first consultation visit with your fertility specialist.
If you would like more information about the benefits and limitations of PGT, you can call PCRM to make an appointment to talk to our genetic counselors. For more detailed information regarding PGT, you can also refer to the web sites of Cooper Genomics and Natera, the laboratories to which PCRM sends biopsied cells for testing.
Types of PGT
Altogether, there are 46 chromosomes (23 pairs) in the human cell. Not too long ago, the standard PGT-A analysis was able to examine only a few chromosomes. Advancing genetic technology now allows scientists to assess the entire set for missing and extra whole chromosomes as well as large missing and extra pieces of chromosomes (deletions and duplications).
Evaluating an embryo’s chromosome make-up using PGT-A techniques helps to select the ones that have the desired number of chromosomes. Transferring these embryos may result in higher implantation and pregnancy rates and may help avoid miscarriage.
There are a number of different PGT-A platforms including array CGH (comparative genomic hybridization), SNP (single nucleotide polymorphism) array and NGS (next generation sequencing). Each technique ultimately performs the same task. All are highly effective, with accuracies of 97-99%.
For the majority of patients undergoing PGT at PCRM, the biopsied cells are sent to Cooper Genomics. Cooper Genomic’s PGT-A method is an NGS platform. The results provide highly accurate information for all 46 chromosomes.
Frequently Asked Questions
At what age should women consider PGT-A?
PGT-A is most effective for women over the age of 37 – 38 years old. This is because women are born with all of their eggs, and as they age, the eggs are more prone to making genetic mistakes. For example, a 30 year-old woman should expect about 25% of her embyros to be aneuploid (genetically abnormal) versus a 41 year-old woman should expect 70% or more of her embryos to be aneuploid. PGT-A can help to screen a group of embryos to find the one with the highest potential for implantation and healthy pregnancy.
How much does PGT cost?
The added cost of a PGT cycle can range from $3000 – $7000+ in addition to a standard IVF + ICSI cycle. This includes the embryo biopsy fee, the cost (billed per embryo) of genetic testing which is paid to the genetics company, and the cost of a frozen embryo transfer.
I am 40 years old with very low ovarian reserve, should I do PGT-A?
This is a very common question and the answer varies based on the individual, or couple’s, objectives. On one hand, many couples desire to know the genetic makeup of an embryo before it is transferred, which can help to achieve a successful pregnancy faster by avoiding failed transfers and miscarriages. However, on the other hand, when there are only 1 or 2 embryos to choose from, a person might choose to transfer those embryos in a fresh cycle rather than incur the extra costs of testing them. In general, the most common reason an embryo fails to implant is due to genetic abnormalities, so the large majority of abnormal embryos will either not implant, or miscarry early in pregnancy.
What if I do PGT-A and there are no euploid (normal) embryos to transfer?
Unfortunately this can happen. Sometimes we test a group of embryos and all of them are found to be aneuploid (abnormal). In this circumstance we recommend not to transfer those embryos and your physician will discuss either attempting another IVF cycle or pursuing alternative options, such as donor eggs.
Can PGT-A ensure a healthy baby?
PGT-A can test for the number of chromosomes in an embryo. PGT-M can test for genetic diseases, for example if one of the parents is a carrier of the disease. Neither PGT-A, nor PGT-M can guarantee a healthy baby. This is because PGT cannot rule out rare genetic diseases or random genetic or developmental differences. There are also developmental differences, such as autism, that are considered to be multifactorial and for which no distinct genetic cause has been identified.