PGD – Preimplantation Genetic Diagnosis
Recently, the Society for Assisted Reproductive Technologies (SART) and the American Society for Reproductive Medicine (ASRM) have changed the recommended terms used to describe the testing of embryos:
- Preimplantation testing: A general term which describes the removal of cells from an egg or embryo and subsequent testing. It is subdivided into a few types
- PGT-a – The A stands for aneuploidy. Aneuploid embryos do not have the correct number of chromosomes.
- PGT-m – The M stands for monogenic meaning one gene. This is most commonly used when the parents are carriers for a genetic mutation. Testing is done to reduce the chances of a genetic disease in the fetus.
- PGT-sr – the SR stands for structural rearrangement. Occasionally, parents may have abnormalities in the structure of a chromosome known as a translocation. Translocations can lead to recurrent miscarriages.
- Older terms that you may still see used from time to time include:
- Preimplantation genetic diagnosis: This term was used initially to describe any type of embryo testing. Many people still use this term instead of PGT to refer to any type of embryo testing.
- Preimplantation screening: This was an older method to refer to aneuploidy screening
- Comprehensive chromosome screening: This term was used in the past when technology allowed the screening of all of the chromosomes instead of just a smaller number.
How does PGD / PGT work?
PGD / PGT is made possible through the use of IVF (in vitro fertilization). In short, a woman is first given fertility drugs to stimulate the development of multiple eggs in her ovaries. She is monitored during this time with blood tests and ultrasounds. At the appropriate time, the eggs are removed in a process known as an (oocyte) egg retrieval. Once the eggs are removed, they are inspected under the microscope to determine which eggs are mature and normal appearing. Each of these eggs will then have a single sperm injected into them. This process is called ICSI (intracytoplasmic sperm injection) .
The day after the ICSI is performed; the injected eggs are inspected under the microscope to determine which have fertilized normally. The embryologist looks for two features of a normally fertilized egg: two pronuclei and two polar bodies. The pronuclei represent the chromosomes that came from the sperm and the egg and which now make up the chromosomes of the newly formed embryo. The polar bodies contain extra chromosomes that the egg got rid of.
The fertilized eggs are then placed back into the incubator and allowed to develop. Two days later, the embryos should have reached the 8 cell stage. These cells are called blastomeres. In a normal embryo, each one of these blastomeres should contain identical chromosomal information. Removal of a single cell at this point is in the process is called a blastomere biopsy. (Blastomere biopsy is not recommend anymore since data emerged to show that it reduces the chances that an embryo would implant.
By the fifth or sixth day after fertilization, the embryo should have reached a stage of development called the blastocyst stage. A blastocyst is an embryo which has divided into 100 or more cells. The cells have divided into two groups: the inner cell mass is a small clump of cells which go on to the form the fetus and the trophoblast which makes up the majority of the sphere that comprises the blastocyst. The trophoblast cells may develop into non-fetal tissue such as the placenta or amnionic sac.
Trophoblast cells may be removed in a process called a trophoblast biopsy. This is currently the recommend time to obtain genetic information from the embryo. This information can be used to select which embryos to place into the uterus.
The most common type of preimplantation testing we do is to look at the number of each type of chromosome present. This is called aneuploidy testing. It is also referred to us aneuploidy screening or by the acronym PGT-a. Any couple that is having in vitro fertilization is a potential candidate for aneuploidy testing. In all women, some percentages of the embryos are going to be chromosomally abnormal.
In the past, the method used to test embryos for chromosome abnormalities was known as FISH or flourescent in-situ hybridization. While FISH is still used today for some indications, it is not the best method to assess for numeric chromosome abnormalities. The primary reason is that the number of chromosomes that can be tested using FISH is limited.
More recently, new technologies have been developed that allow testing of all 24 chromosomes in an embryo. The current preferred method is called Next Generation Sequencing (NGS) . Next Generation Sequencing is a more highly automated technology that improves accuracy and reduces the overall cost of testing. Almost all aneuploidy screening in IVF today is performed using next generation sequencing.
A small percentage of couples who have a problem with recurrent miscarriage may themselves have a chromosome abnormality known as a translocation. This is a structural abnormality that occurs between two chromosomes. Preimplantation testing can also be used to identify embryos with translocations. This is now referred to as PGT-SR.
There are other types of problems that can be detected in embryos also. We can perform true preimplantation genetic diagnosis. That is, identify embryos with certain genes or genetic mutations. This is now referred to as PGT-M.
One of the more controversial procedures we have performed is testing embryos to determine whether they are tissue matched to siblings that may be suffering from diseases that could be cured with a bone marrow or stem cell transplant. Another controversial procedure is testing embryos to determine their gender so that a couple can have a child of a particular sex. This is known as gender selection.
Frequently Asked Questions About PGD / PGT
Does performing an embryo biopsy for PGD damage an embryo?
Very, very rarely, an embryo can be damaged by the biopsy procedure. If this occurs, it can be identified right away by viewing it under a microscope.
Does performing an embryo biopsy for PGD / PGT make pregnancy less likely?
There is very good evidence that blastomere biopsy may reduce the chance for implantation compared to a trophoblast biopsies.
I’m under age 35, so my embryos won’t have chromosome abnormalities, right?
Wrong. We and others have studied women of different age groups and have found that even younger women have abnormal embryos. We could identify that in women under 35, 30-40% of the embryos tested were abnormal (too many or too few chromosomes). This does not mean that all women will have 30-40% of their embryos abnormal. Some might have a higher abnormality rate and some lower. Overall, it averages out to 30-40% at that age. We have seen some younger women with recurrent IVF failure have abnormality rates over 90%
What are the chances that an abnormal embryo is going to be missed by PGD / PGT?
Remember that an embryo can have many different types of abnormalities. Preimplantation genetic diagnosis is only going to test for a specific type of abnormality. For instance, testing to determine if an embryo will produce a baby with Down’s syndrome (caused by three copies of chromosome 21) will not rule out the possibility that the embryo also has a gene mutation that would cause the baby to have cystic fibrosis.
What are the chances that an embryo will be diagnosed as abnormal when it is really ok?
When an embryo starts dividing, each of the daughter cells is supposed to be identical to the parent cell. Sometimes, however, the embryo can make a mistake. One cell from an eight cell embryo may be slightly different than the remaining seven cells. This is called mosaicism. Mosaicism can affect the results of PGT.
With Next Generation Sequencing (NGS), the testing laboratory can quantify the percentage of abnormal cells in the embryo biopsy. High level mosaics are those that have >50% of the cells that are abnormal. These embryos are not recommended for transfer due to a lower chance for pregnancy and higher risk for miscarriage. Low and Medium mosaics which have less than 50% abnormal cells may be transferred. Studies which transferred these embryos found that they have the same pregnancy rates and live birth rates as non mosaic embryos.
Will PGD / PGT increase my chance for having a baby?
The answer is… it depends.
Ordinarily, older women have a lower pregnancy rate and a higher miscarriage rate. This is true even when performing fertility treatments such as IVF. With each year, the pregnancy rate declines and miscarriage rate rises. Both problems are primarily due to the higher rate of chromosomally abnormal embryos that occur in older women. It makes sense that by finding and placing the normal embryos into the uterus, the chances are better that a delivery will occur.
Some studies have now demonstrated that for patients over the age of 35, the use of trophoblast biopsy along with Next Generation Sequencing , can improve the live birth rate.
It is possible that younger women may also benefit from PGT but since they have a lower percentage of abnormal embryos, the benefit is likely to be smaller. Therefore, a much larger number of women need to be studied in order to statistically prove an effect.
Is it likely that insurance will cover the cost of PGD / PGT?
It is unlikely (though not impossible) that PGD / PGT will be covered by your insurance. Most insurance companies still consider PGD / PGT to be experimental even though we have been doing this since the early 1990s.
Don’t look for this to change any time soon. Although we have a law in Illinois which requires most employers to cover infertility, it took a great deal of effort to get that law passed.
PGD / PGT is a much more controversial technology than IVF. It can be used for things such as gender selection and selection of embryos for tissue typing. Many people do not believe that these technologies should be allowed. Because of this, there are not likely to be politicians that are going to be willing to back a measure that will require employers to cover PGD.
Type of biopsies used for preimplantation testing
- Polar Body Biopsy The polar bodies can be removed after fertilization and used for preimplantation testing. Not commonly used anymore.
- Blastomere Biopsy After the embryo has divided into eight cells (blastomeres), one of the cells can be removed and used for preimplantation testing. Not commonly used anymore.
- Trophoblast Biopsy After the embryo has reached the blastocyst stage, several cells that are not part of the fetus can be removed and used for preimplantation testing.
Types of Preimplantation Testing
- Abnormalities in chromosome number to improve the IVF pregnancy rate and decrease miscarriage risk
- Abnormalities in chromosome structure to reduce the risk for recurrent miscarriage
- Genetic mutations
- Hereditary Cancer
- PGD for miscarriage
- Tissue matching (HLA typing)
- Gender Selection
Types of Preimplantation Laboratory Techniques
- Flourescent in-situ hybridization – FISH
- Polymerase Chain Reaction – PCR
- Comparative Genomic Hybridization – CGH
- Next Generation Sequencing (NGS)
Preimplantation Genetic Diagnosis: PGD Pictures of normal embryos
- Polar Body biopsy with two color FISH
- Polar Body biopsy with three color FISH
- Blastomere biopsy with three color FISH
Preimplantation Genetic Diagnosis: PGD Pictures of abnormal embryos
- Polar body: FISH: Monosomy 21
- Polar Body: FISH: Trisomy 21
- Blastomere: FISH: Triploidy
- Blastomere: FISH: Trisomy 21 (Missing)
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