PGD – Preimplantation Genetic Diagnosis
Recently, the Society for Assisted Reproductive Technologies and the American Society for Reproductive Medicine 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 work?
PGD 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.
At this time, the embryologist can perform a polar body biopsy or removal of the polar bodies from the embryo. The fertilized eggs (without the polar bodies) are then placed back into the incubator and allowed to develop. Two days later, the embryos can be removed and inspected again. If the embryos are viewed at this stage, it is hoped that they 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. This is the second point at which genetic material can be removed from the embryo. Removal of a single cell at this point is in the process is called a blastomere biopsy. (Blastomere biopsy is not commonly performed anymore.)
Again, the embryo is returned to the incubator for further growth and development. 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 represents the third opportunity to obtain genetic information from the embryo. At any of the above stages, the cells that were removed from the developing embryo and specifically the genetic material inside of the cells can be tested for various abnormalities or characteristics. 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. One method is called CGH or comparative genomic hybridization. CGH can by performed on a specially designed chip and analyzed with a computer to interpret far more information than could be accomplished with the human eye alone.
NGS or 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
Does performing an embryo biopsy for PGD damage an embryo?
In the right hands…No. We have now studied thousands of embryos. Compared to IVF without PGD, embryos that have a polar body biopsy or embryo biopsy develop in a similar way. For example, when we compare the percentage of eggs that achieve normal fertilization, the PGD embryos which had a polar body biopsy had a normal fertilization rate of 78%. The eggs that did not have a PGD biopsy had a normal fertilization rate of 76%. Cleavage rate is a measure of the percentage of fertilized eggs that go on to start dividing. PGD embryos have a cleavage rate of 96%, compared to embryos without PGD which divide 95% of the time. A very important quality measure is how often a fertilized egg will become a blastocyst. PGD embryos will develop into blastocysts about 40% of the time. Non-PGD embryos become blastocysts slightly more often at 47%, a difference that is not statistically different. Rarely, an embryo can be damaged by the biopsy procedure itself. If this occurs, it can be identified right away by viewing it under a microscope.
Does performing an embryo biopsy for PGD make pregnancy less likely?
There is very good evidence that polar body biopsy does not affect the chance for an embryo to implant and form a normal baby. There is however, 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, 40% of the embryos tested were abnormal (too many or too few chromosomes). This does not mean that all women will have 40% of their embryos abnormal. Some might have a higher abnormality rate and some lower. Overall, it averages out to 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?
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 PGD or PGS especially when blastomere biopsy is performed.
Remember, during a blastomere biopsy, one cell in an eight cell embryo is removed and tested. It is assumed that this cell is going to be representative of the entire embryo. If it is not, then a misdiagnosis can result. Trophoblast biopsy will reduce the chance for misdiagnosis due to mosaicism. Next Generation Sequencing may also reduce the risk.
Studies which used different technologies to test the same embryos have on occasion shown different results. The percentage of these discrepancies is low.
Will PGD increase my chance for having a baby?
The answer is as yet unknown.
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.
Preimplantation genetic diagnosis using FISH has been shown in our studies and in studies from other medical groups to increase the chance for pregnancy and reduce the risk of miscarriage in women who are 37 or older. Several larger studies using FISH, however, failed to find a benefit.
Several recent small studies have shown a benefit to PGD with CGH. These studies showed that using 23 chromosome analysis of embryos eliminated the decrease in live birth rates associated with increased age. To date, there are no large scale studies which have compared live birth rates rates using 23 chromosome testing versus no testing.
It is possible that younger women may also benefit from PGD 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?
It is very unlikely that PGD will be covered by your insurance. Most insurance companies still consider PGD to be experimental even though we have been doing PGD 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. Even then, it was passed with some major loopholes that allow some employers to deny coverage to infertile couples.
PGD 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.
- Blastomere Biopsy After the embryo has divided into eight cells (blastomeres), one of the cells can be removed and used for preimplantation testing.
- 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)