Preimplantation Genetic Screening
A Guide to Understanding Preimplantation Genetic Screening
One thing you’re guaranteed to encounter when you receive an infertility diagnosis is information overload. Whether you get it from your doctor, the Internet or a well-meaning friend who has gone through the same thing, you are going to be bombarded with information and vocabulary that can be unfamiliar and overwhelming.
When it comes to navigating the topics and terms being thrown at you, it can be easiest if you break it down into more manageable pieces.
If you’ve opted to undergo in-vitro fertilization (IVF), one of the things your doctor may have talked to you about is preimplantation genetic screening or PGS. Doctors are using PGS to determine the viability of an embryo prior to implantation, as well as to give couples more information about the chromosomal makeup of their embryos.
While there are a number of benefits to PGS, the science behind it can be rather involved and confusing, especially if you’ve never heard of it before. Before you move forward with the testing, it can be helpful to understand what you’re getting in to.
What Is Preimplantation Genetic Screening?
PGS is a test performed on an embryo prior to IVF to detect any genetic abnormalities that may be present. This is done by extracting a couple of cells from the embryo and then testing them to determine whether or not the embryo has the correct number of chromosomes. This screening process has been developed to increase the chances of a healthy embryo being implanted into a woman’s uterus during IVF by identifying an embryo with too many or too few chromosomes before a woman has undergone the implantation process.
There are a number of reasons that a woman can opt to undergo PGS testing prior to IVF. It is especially helpful for couples who have had several failed pregnancies previously or if they have a genetic risk of chromosomal abnormalities. However, before undergoing the procedure, it’s important to understand the pros and cons of PGS.
How Is PGS Performed?
When a couple opts to undergo IVF, eggs are retrieved from the woman — or an egg donor — and then combined with sperm in the lab. Once an egg is fertilized, it is allowed to mature for several days, and then PGS is performed by performing a biopsy to retrieve two or three cells. Those cells are then analyzed to determine the chromosomal makeup of the embryo. By doing this, doctors can determine whether an embryo is missing chromosomes or has too many chromosomes — both conditions that could complicate implantation and subsequent pregnancy after IVF.
Pros of PGS
There are several reasons many women opt to have their embryos undergo PGS prior to IVF:
Insight Into the Reasons for Infertility
Sometimes couples are unable to get pregnant or have reoccurring miscarriages for reasons that cannot be explained. When they opt to undergo PGS, the genetic screening can sometimes provide insight into why an embryo is not able to implant into a woman’s uterus or why they have not been able to sustain a pregnancy. In other words, in certain cases, PGS can provide a more detailed look at what is causing a woman’s infertility than other standard tests.
A Chance to Prepare for a Baby With Genetic Abnormalities
By identifying problems before implantation, a couple can have advanced warning of the difficulties and complications they will encounter throughout their pregnancy and after birth. While some couples do opt to forego IVF when problems are identified, others opt to continue with the process. When they choose to proceed with IVF, knowing about potential complications can ease some of the uncertainty as they wait to determine whether implantation — and ultimately the pregnancy — is successful.
Increases the Chances of a Successful IVF Implantation
Gaining insight into the genetic composition of an embryo prior to implantation can help doctors and couples determine how successful their implantation will be in advance. While there continues to be research into the intricacies of the process, recent findings have shown that PGS results in healthier embryos being implanted and, therefore, a greater chance of success in a woman becoming pregnant.
Cons of PGS
Some of the cons of PGS include the following:
Results From PGS Can Be Limited
Because only a select few cells are tested, there can be times that the results of a PGS don’t tell the whole story. Couples who opt to have PGS done before IVF should work closely with their doctor to understand the results and what they will mean for the success of their IVF procedure, as well as any subsequent pregnancy that results.
As with any scientific endeavor, there is a margin of error, although ongoing improvements in the PGS process have shown improvements in results and a decrease in the room for error. However, because PGS only surveys a small sample of cells, in a small percentage of cases, conditions such as mosaicism may cause results to be skewed.
PGS Doesn’t Identify Genetic Predispositions
PGS is only used to identify chromosomal abnormalities. It does not identify an embryo’s predisposition to certain other genetic conditions or diseases. If there is a reason to believe that your embryo is at risk for developing a genetic condition, such as sickle cell anemia or cystic fibrosis, couples can opt for another procedure called preimplantation genetic diagnosis (PGD). Since it is less likely for a couple to create an embryo that is at risk for a single-gene disorder, this test is not as widely recommended. That being said, if you and your partner have reason to be concerned, you may want to talk with your doctor about whether PGD is necessary.
How Accurate Is PGS?
The research behind PGS is continually growing and expanding as researchers find more ways to apply this procedure to the process of conception and treatment of couples struggling with infertility issues. When PGS was first used, it was not shown to have a significant impact on pregnancy rates among IVF recipients. Some studies even found that it slowed down the embryo’s ability to grow and implant in a woman’s uterus.
In recent years, the process of PGS has been improved and the results speak for themselves. The success of IVF after an embryo has undergone PGS is increasing and the new method of testing does not harm the embryo or endanger its chances of implanting successfully.
One of the current limitations with PGS is that testing such a small sample of cells at such an early stage of an embryo’s development may not provide doctors and couples with the full picture of an embryo’s genetic and chromosomal makeup. As with any scientific process, there is a margin for error.
What Is the Difference Between PGS and PGD?
When the topic of PGS comes up, it may also lead to a discussion about Preimplantation Genetic Diagnosis (PGD). Like PGS, PGD is a procedure that is performed on an embryo to determine the risk of certain complications. Unlike PGS, PGD is only performed when a couple is believed to have a high risk of passing on genetic disorders or diseases.
Like PGS, PGD is performed by extracting cells from an embryo about five days after it has been fertilized in the lab. However, this is where the similarities end. In PGS, the chromosomes are counted. In PGD, it is the DNA that is analyzed for abnormalities. To break it down a little more, PGS is done to determine the viability of the embryo, whereas PGD generally tells more about the genetic makeup of the child that may result from the embryo.
Because most couples are not at risk for single-gene disorders — those detected by PGD — these highly expensive tests are not as widely recommended as PGS. However, they may be appropriate for women and/or couples with the following conditions:
- Those with chromosomal disorders
- Carriers of single-gene disorders
- Carriers of genetic, sex-linked disorders
- Women over 35
- Couples who have had reoccurring pregnancy losses
- Women who have had one or more failed infertility treatments
There is some controversy surrounding PGD because it has been used in the past for the process of gender selection. However, this is not the purpose it was intended for and is not a procedure that should be undertaken lightly. It is also important to remember that PGD does not always reveal the full story of an embryo’s genetic makeup. After implantation is successful, prenatal testing is still recommended. And finally, it is important to remember that there are certain genetic conditions that do not reveal themselves until later on in life, meaning that PGD may not provide the full picture of what a couple’s potential child may face as they grow.
Both PGS and PGD, when used, should be considered as tools to be used in the IVF journey. But, as a carpenter uses many tools to build a house, a couple should rely on a variety of tools to make informed decisions along the path of their IVF journey. Your doctor and team of reproductive specialists can help you navigate the process and understand how these tools may benefit you along the way.
While PGD does have many positive benefits, it has also developed a bit of a reputation of providing a more ethical dilemma because, in the past, it has also been used for the purposes of gender selection, which has generated an ethical debate among many people.
When used appropriately, tests like these are seen as a chance for couples to prepare themselves for whatever lies ahead — both in the potential for success or failure of implementation and the challenges they may face in pregnancy and beyond. Not only that, but PGS, specifically, can provide insight into why a couple has struggled to conceive or stay pregnant, and it can give them a glimpse into challenges they may face if IVF does result in a successful pregnancy.
What Is Mosaicism and How Does It Impact PGS?
One of the challenges associated with the use of PGS is the condition known as “mosaicism.” Mosaicism is a condition in which cells within a single embryo have a different makeup. This is the result of a cell division error that occurs very early on in the development process. The reason we mention this is because, in about five percent of cases, mosaicism can skew the results of PGS. While five percent is a relatively small percentage, it does come up and is worth talking about.
The results of PGS can be skewed by mosaicism because, when this condition is present, the cells that were tested are not like some of the other cells in the embryo. That means there cannot be as clear of an understanding of the condition of the embryo and what (if any) complications may arise. While embryos that are identified with mosaicism are considered to have a lower chance of success implanting into a woman’s uterus, this is not always the case. There are times when they are implanted successfully, but it is important to understand the statistical risks associated with moving forward in the process. Not only is the chance of implantation lower, but if the embryo is to become successfully implanted in the uterus, there is a higher risk for a miscarriage.
While some couples may opt to use another embryo that does not show evidence of mosaicism, some couples choose to move forward with IVF. While a couple should not necessarily opt to forgo IVF if an embryo is identified with mosaicism, understanding what this condition means for the embryo and its viability can help you to make wise decisions about your next step. It can also help you approach your procedure with caution while you wait to see what will happen. There is no black-and-white answer in this case.
As with any of the tests and procedures surrounding IVF, asking questions is a valuable part of the process. Don’t be afraid to talk with your doctor and make sure you understand what is happening. While procedures like PGS can be a valuable part of the IVF process, they can also be confusing and overwhelming.
No one should try to navigate this alone. At Western Fertility Institute, our goal is to provide a warm, welcoming environment where our clients can feel comfortable asking questions and sorting through all of the information they have been given. We want to walk with you throughout the whole process, from the informational stage all the way through PGS and IVF. Knowing that these procedures are seeing increasingly positive outcomes, we also look forward to walking with you through your pregnancy journey.
Whether you are just beginning to consider building your family or you have been trying for quite some time, we want to help you achieve your dreams. Let us join in your journey to completing your family. Contact us today.