If you are considering going through the process of in-vitro fertilization (IVF) then it is likely you have heard about the option to genetically test your embryos. This process is known as preimplantation genetic testing (PGT) and can be done to screen embryos for chromosomal abnormalities (known as PGT-A) or can be done to screen embryos for a specific genetic disease if both parents carry a gene for the same disease (known as PGT-M).
What are some reasons to consider PGT?
- To test embryos for chromosomal abnormalities
- If you and your partner both carry the same genetic disease
- Female aged over 37 to reduce the miscarriage rate
- Sex selection for family balancing
The following information may help guide women in their discussions with their fertility doctors on whether PGT may be an option that is right for them.
What are chromosomes and what is their relation to PGT?
Chromosomes contain all of our genetic information and each person has two pairs of 23 chromosomes for a total of 46 chromosomes. We inherit one set of 23 chromosomes from our mother and the other set of 23 chromosomes from our father. When the egg and sperm meet, they each contain 23 chromosomes. If an egg or sperm contains more or less than 23 chromosomes, the resulting embryo will contain more or less than 46 chromosomes. This can lead to conditions such as Down Syndrome (having an extra chromosome 21) or other genetic conditions. PGT-A is a technology that allows us to test the number of chromosomes in an embryo to make sure they contain the correct number.
When is genetic testing done during the IVF process?
Genetic testing of the embryos is done when the embryo reaches the blastocyst stage, typically on day 5 or day 6 after the egg retrieval. On these days, embryos that have developed normally to this point are biopsied. The biopsy involves removing 3-5 cells from the outer layer of the embryo, known as the trophectoderm, which is what will eventually develop into the placenta. These cells are then sent to a genetic reference laboratory and genetic results are generated for each embryo that was tested and results are provided back to the physician and patient. From there, you will learn how many embryos tested normal vs. abnormal.
Female age and its relation to “normal” embryos
Female age has perhaps the biggest impact on the number of normal embryos a woman will produce. This is due to the fact that we know egg quality is highly associated with female age and as women ages, egg quality will start to decline and the likelihood for chromosome errors in her eggs will increase. PGT-A allows for embryos to be screened prior to transfer with the intention of improving pregnancy rates by selectively transferring a normal embryo, increasing the chance of pregnancy per embryo transferred and reducing the miscarriage rate. However, women over 40 or those who have a diminished egg quality may need to have many eggs available to get to that one normal embryo. This may mean multiple egg retrievals depending on your ovarian reserve.
Besides testing for chromosome number, what else is PGT useful for?
- Screening out embryos that may contain genetic diseases. PGT may be recommended to couples if they are both carriers for the same recessive disease. An example is cystic fibrosis (CF). In order for a baby to be born with CF, they must inherit two copies of the CF gene, one from their mother and one from their father. The mother and father in this case are both carriers of the disease. They themselves are not impacted by carrying the disease, but if they have a baby who inherits both copies, the baby will be affected. PGT can allow for the testing and selection of embryos that are not affected by the disease if both parents happen to carry the same genetic disease.
- Some studies suggest that women who elect to genetically test their embryos and subsequently undergo the transfer of a normal embryo may have a faster time to pregnancy and lower chance of miscarriage compared to women who do not genetically test their embryos.
- Family balancing. PGT may be used by some couples to allow for sex selection of the embryo that is transferred for family balancing.
- Increasing the use of single embryo transfer, thus decreasing the number of embryos transferred at one time and reducing the incidence of twins and higher order multiples that was seen with IVF in the past prior to this technology being available.
What are the potential cons to genetically testing embryos that I should be aware of?
- Cost. Genetic testing adds an additional cost to the IVF procedure and may be anywhere from several hundred to a few thousand dollars. The cost typically varies based on the testing company your fertility clinic uses as well as the number of embryos tested.
- Damage to the embryo. While one study demonstrated that embryos biopsied at the blastocyst stage (day 5 or 6 of development) have a lower chance of damage than if they were biopsied at the cleavage stage (day 3 of development), we still can’t stay with 100% certainty that no harm comes to embryos in the testing process.
- Results are not 100% accurate. While the accuracy of PGT-A is upwards of 98% accurate, it is not 100%. Even if you become pregnant with a normal embryo, it is still recommended that you discuss routine prenatal testing with your OB-GYN and see which additional tests you may need.
- No diagnosis or mosaicism results. Some embryos screened may return as no diagnosis (this is a rare diagnosis) meaning there were not enough cells to properly analyze the embryo. You are then faced with a choice of doing a re-biopsy of the embryo and possibly subjecting it to further damage or potentially transferring an embryo that could be abnormal. Mosaicism of the embryo means there are a mixture of normal and abnormal cells in the embryo and the embryo will be classified as having an unknown reproductive potential
Can PGT screen my embryos for autism?
This is a common question we are asked, but the answer is no; genetic testing of embryos cannot screen for autism. The reason being that the underlying cause of autism is still unknown, but is thought to be multifactorial. This means that a variety of genetic and environmental factors all likely contribute to the development of autism.
If I transfer a “normal” embryo, is my chance of pregnancy 100%?
No, unfortunately the chance of pregnancy is not 100%. The reason is that there are many other factors involved in the successful implantation on an embryo besides genetics. These factors include the transfer technique and any complications there, as well as the uterine microenvironment. So while genetic testing may improve your chance of pregnancy it will not be 100%.
Is genetic testing of my embryos something I should consider?
This is a personal decision and it is best made with your fertility doctor after reviewing your medical history and determining your needs. There is no right or wrong answer and whether someone should go forward with testing their embryos is ultimately up to them. But, it is important for all women to be aware of what genetic testing is as well as the pros and cons so they have the knowledge to make an informed decision for their fertility journey.