PGD & PGS Genetic Testing
PGD & PGS genetic testing at a glance:
- PGD (pre-implantation genetic diagnosis) is a technique used to test the embryos for genetic or chromosomal disorders prior to transfer into the uterus during in vitro fertilization (IVF).
- By contrast, PGS (pre-implantation genetic screening) is the evaluation of embryos for multiple abnormalities at once.
- PGS is performed on embryos of parents with no known genetic defects, as opposed to PGD which evaluates for a known or suspected genetic defect.
- PGD & PGS genetic testing may prevent the transmission of certain heritable genetic and chromosomal disorders such as cystic fibrosis and Fragile X, among others.
What is PGD?
Pre-implantation genetic diagnosis (PGD) is a technique used during in vitro fertilization (IVF) procedures to test the embryos for genetic or chromosomal disorders prior to transfer into the uterus. This testing may prevent the transmission of certain heritable genetic and chromosomal disorders.
By contrast, PGS (pre-implantation genetic screening) is the evaluation of embryos for multiple abnormalities at once. The purpose of PGS is to increase the chance of a live birth, not to diagnose known diseases.
Who is a candidate for PGD?
PGD is indicated for couples that are known to be carriers for a specific genetic disorder. Disorders successfully prevented by PGD include:
- Cystic fibrosis
- Huntington’s disease
- Sickle cell disease
- BRCA1 mutations (a cancer causing gene)
- Fragile X
- Myotonic dystrophy
Some couples also want PGD for family balancing, to increase the chance that they will conceive a girl or a boy.
Rebecca and Jeff had 7 embryos; PGS showed only 3 were normal.
How is PGD performed?
To accomplish PGD, embryos are obtained through an IVF cycle. One to two cells are biopsied from the embryo and evaluated. A diagnosis is made within a day of the test; only unaffected embryos are replaced into the uterus.
The two most common lab techniques utilized during a PGD cycle are fluorescent in situ hybridization (FISH) and polymerase chain reaction (PCR). FISH is used to study chromosome numbers, and in some cases, structure. PCR is used to detect single gene (monogenic) disorders. DNA from the embryo is studied to determine the presence or absence of the mutation.
What are the risks of PGD?
The goal of PGD is the birth of an unaffected, healthy child. The PGD procedure itself has not been linked to any birth defects. However, no technology is perfect. It is possible that embryos could be damaged by the PGD biopsy. If the embryos stop growing, it’s difficult to know how they may have fared if no biopsy had been performed.
In some cases, embryos can be misdiagnosed. Specifically, when testing for specific gene disorders, the chance of transferring an affected embryo that was mistakenly identified as normal is between two and eleven percent, depending on the specific disorder being tested for.
How much does PGD cost?
PGD adds additional cost to the IVF procedure, ranging from $4,000-6,000. Other incidental costs for embryo shipping may be incurred.
What Is PGS?
Pre-implantation genetic screening (PGS) is the testing of embryos for multiple genetic disorders at the same time. PGS is performed on embryos of parents with no known genetic defects, as opposed to PGD which evaluates for a known or suspected genetic defect. We at TRM use next generation sequencing technique which has thus far proven to be the most reliable technique we have used to date.
The premise behind PGS is that we can identify embryos that are aneuploid, meaning they contain the wrong number of chromosomes in a cell, either too many or too few. By doing so, we can determine which embryos are aneuploid and have no chance of resulting in the live birth of a healthy child.
PGS helps us avoid transferring embryos that might get a woman pregnant, only to make her suffer a miscarriage.
PGS does not improve the quality of any embryo we biopsy; however, it does give us important information about it.
When we transfer embryos that are genetically normal, we maximize the chance of pregnancy while minimizing the chance of miscarriage or of having a baby with a genetic syndrome such as Down syndrome.
How can PGS benefit patients?
PGS is available to all patients. It is a more reliable way to determine the quality of an embryo than by simply looking at the embryo’s shape (morphology) or grade.
PGS helps with embryo selection
As women get older, a higher and higher percentage of their embryos will be genetically abnormal. Therefore, if we don’t do PGS, we typically transfer more embryos into women as they age.
For example, if we select an embryo based on shape alone, a top-quality embryo in a woman 32 years old or younger generally has about a 60-65 percent chance of resulting in a pregnancy. If the embryo is genetically normal, the chance goes up to 80 percent.
A top-quality embryo in a 38-year-old woman has about a 35 percent chance of resulting in pregnancy. However, if the embryo is genetically normal, then it too has about an 80 percent chance of pregnancy.
So, no matter what her age, if a woman has PGS performed, she can confidently transfer a single embryo into her uterus and expect a high rate of pregnancy with a low rate of twins.
PGS reduces the chance of miscarriage
The vast majority of genetically abnormal embryos will not result in a live birth. However, they CAN get a woman pregnant, only for her to suffer a miscarriage.
If we do not transfer genetically abnormal embryos, we greatly reduce the risk of miscarriage with IVF (in vitro fertilization).
PGS can tell if none of the embryos are suitable to transfer
There are patients who generate multiple embryos, yet none of them are euploid (genetically normal). If a woman generates seven embryos and does not know they are all abnormal because they have not been evaluated by PGS, then she may go through multiple embryo transfers and multiple miscarriages. What she really needed all along was to get a new batch of eggs.
This is an example of where PGS is very cost effective. The cost of an embryo transfer and the costs associated with many miscarriages will exceed the cost of PGS.
PGS is particularly helpful as women get into their later 30s, since fewer and fewer of their embryos are normal. It can be particularly useful for these women to know if they should go through IVF again to get a new batch of eggs.
Just because a person does not have any normal embryos in one batch, does not mean they won’t have any normal ones from a second attempt.
PGS can help with family planning
If a person would like two or three children, it would be nice to know if her batch of embryos has that many normal ones so she can freeze normal embryos for future pregnancy. If not, she might consider another IVF stimulation to increase the number of normal embryos. The younger a woman is when she stimulates, the greater her chances of having a normal embryo.
Conversely, if a person already has two children and has four embryos she has not used, it might be very helpful to know which, if any of them are genetically normal. People are more likely to abandon their embryos if they have a large number of them.
However, if they did PGS, they might learn that only one or two of those embryos are normal. Knowing only one or two more pregnancies could result from their batch of embryos will often give couples the courage to use all their embryos.
Risks of PGS
The estimate is that 1 in 100 embryos will be lost or damaged by doing PGS. PGS also requires freezing of the embryos, and not all embryos survive the freeze/thaw processes. Fewer than 5 percent of embryos will have some damage from the freezing or thawing. So PGS is not risk free to the embryo; however, the vast majority do well.
Performing PGS on an embryo does not improve the embryo’s quality or its ability to result in a pregnancy. If you have only one embryo, for example, doing PGS will not help you get pregnant. By doing PGS, you know more about the embryo and only learn if it is one that can result in a healthy live birth.
Trends in PGS
More and more couples are opting for PGS, as it helps them confidently select a single embryo for transfer with a very high rate of success and low chance of miscarriage. Couples are also choosing to do PGS more because it means they will need fewer frozen embryo transfers in the future in order to utilize all of their embryos.
For patients with many embryos, this can be cost effective. For older women, it can be critical to let them know if none of their embryos will work and instead of doing any transfers, they need a new batch of embryos.