Advanced reproductive technology enables parents to reduce the risk of genetic disease, chromosomal abnormalities, and related complications during pregnancy, using pre-genetic carrier screening and/or preimplantation testing of the embryo.
Pre-genetic carrier screening vs. preimplantation genetic testing: What’s the difference?
Pre-genetic Carrier Screening is performed on parents prior to or during pregnancy to test for genetic disorders that could be passed on to their child. Once collected, DNA samples are tested in a lab for genetic mutations (changes in the DNA) that are associated with specific diseases.
Preimplantation Genetic Testing (PGT) is performed on embryos that were created through in vitro fertilization (IVF) to help parents choose the healthiest embryo for implantation and reduce the risk of either chromosomal abnormalities or transmission of any genetic diseases identified through an earlier carrier screening.
What are the different types of genetic tests and what do they test for?
Pre-genetic carrier screening
Pre-genetic carrier screenings test for single gene mutations which could result in the transmission of one of three kinds of single gene disorder:
- Autosomal recessive disorders, which result from gene mutations on chromosomes 1–22 and are passed to the child only when both parents are carriers (Most single gene disorders are autosomal recessive.)
- Autosomal dominant disorders, which result from gene mutations on chromosomes 1–22 but are passed from just one parent to the child
- X-linked disorders, which result from gene mutations on the sex chromosomes (X) (Because females have two copies of the X chromosome but males only have one, X-linked disorders are more common in males.)
Carrier screening is optional, and parents can choose to either test for specific high-risk disorders or get an extensive carrier screening for hundreds of different diseases. Some of the most common single gene disorders are:
It is important to note that a positive screening result does not necessarily mean that a parent has that genetic condition, nor that their child will inherit the disease. However, the presence of these mutations increases the risk of inheritance: if both parents are carriers, there is a 25% chance that the child will have that disorder and a 50% chance that the child will be a carrier.
Preimplantation genetic testing
There are three different types of tests that can be performed on the embryo prior to implantation.
Preimplantation genetic testing-monogenic (PGT-M)
PGT-M tests specifically for single gene (or monogenic) disorders. Many parents use PGT-M to further test for the presence of any high-risk disorders found during pre-genetic carrier screening.
Preimplantation genetic testing-aneuploidy (PGT-A)
PGT-A tests for aneuploidy, a type of chromosome disorder in which the embryo is not 23-chromosome competent.
An embryo is not 23-chromosome competent when it has an extra (trisomy), missing (monosomy), or partially missing chromosome.
Examples of aneuploidy include:
- Trisomy 21 (Down syndrome)
- Trisomy 18
- Trisomy 13
Preimplantation genetic testing-structural rearrangements (PGT-SR)
PGT-SR tests for all types of chromosomal structural rearrangements, including:
- Ring chromosomes
Who should get genetic testing?
Pre-genetic carrier screening and preimplantation testing can help parents make informed decisions about how they build their families. Some parents, even those who don’t suspect heightened risks, choose to undergo genetic tests simply for the added assurance of a successful, healthy pregnancy. Other parents intentionally select tests based on risk factors like personal or family health history, ethnicity, or pregnancy history.
Who should consider carrier screening?
Anyone who is pregnant or considering becoming pregnant can choose to receive carrier testing to better understand their genetic risks. Some specific reasons parents choose carrier screening include:
Personal or family history
Those with a known history of a specific genetic disorder, such as a hereditary cancer, often choose screening to better understand their family-building options prior to conception. For example, those with a strong family history of breast cancer may select to specifically test for the BRCA mutations most commonly associated with breast cancer.
While no genetic disorder is exclusive to a specific population, some disorders occur at higher rates among certain ethnic populations. For example, cystic fibrosis is more common among people of European descent, and sickle cell anemia occurs more frequently in people of African descent. Many parents choose to test for just the disorders most common among their ethnic group.
Who should consider PGT for monogenic disorders?
Parents who received positive results in their carrier screenings can reduce the risk of having an affected child by testing the embryos’ carrier status for that specific disorder. This allows parents to select the healthiest embryo for transfer. PGT can also be used to determine the sex of the embryo, which can be helpful for couples who are carriers of sex-linked genetic disorders.
Who should consider PGT for aneuploidy and structural rearrangements?
When undergoing IVF, there are a few factors that often encourage patients to test their embryos for aneuploidy:
The risk for chromosomally abnormal embryos increases with age. Typically, PGT is recommended when the woman supplying eggs is 35 years of age or older.
History of miscarriage or stillbirth
When it comes to IVF, chromosomal abnormalities are the most common cause of miscarriage. For this reason, PGT is commonly recommended for women with a personal or family history of miscarriage or stillbirth.
Reasons to get genetic testing
Deciding whether or not to undergo these tests is a complex decision that depends on a range of individual circumstances, personal values, and family-building goals. To help you make this decision for your family, here are some reasons that parents commonly choose to get testing:
Confidence in chromosomal normalcy
By screening embryos for genetic abnormalities before implantation, PGT allows parents to select only those embryos that are free of chromosomal abnormalities, increasing confidence in the health and quality of life of their future child.
Decreased miscarriage rates
Because it has been shown to significantly increase live birth rates, many parents choose PGT to improve their chances of success even if they don’t have a personal or family history of miscarriage or stillbirth.
Reduced risk of genetic disease
Carrier screening enables parents to understand their personal risk for suspected genetic disorders, and PGT allows parents who have identified a high risk to take action toward preventing an affected pregnancy. With PGT-M, our physicians at ICRM have helped numerous couples go home with a healthy baby—even when both parents were carriers for the same genetic disease.
Improved efficiency & lower cost
One reason many parents choose to undergo PGT is to improve the efficiency of the IVF process. By selecting only the embryos that are free of certain genetic conditions, PGT can increase the chances of a successful pregnancy and help prevent the need for multiple rounds of IVF. This not only can save parents time and money, but the emotional toll of failed rounds of IVF.
What can I expect of pre-genetic testing?
Carrier screenings are quick and simple, requiring either a blood test or a saliva sample. At ICRM, we typically collect blood samples, although our out-of-town patients have the option of collecting their own sample with an at-home saliva collection kit. Once collected, DNA samples are sent to a lab where they are screened for all disorders requested by the patient.
PGT involves a biopsy of the embryos at their blastocyst stage: 5–10 cells are collected from the trophectoderm cells—these are cells that become the placenta, not the fetus. In the lab, these cells are tested for their genetic carrier status and/or their chromosomal competence. This test is safe, and it is not considered harmful to the embryo.
Pre-genetic testing at ICRM
At Idaho Center for Reproductive Medicine, we offer both full-range pre-genetic carrier screening and preimplantation genetic testing, including PGT-A, PGT-M, and PGT-SR, to help parents make informed decisions about their family planning.
If you’re not sure if genetic testing is right for you, schedule a consultation with our physicians. We can help you understand the genetic risks and benefits of testing under your specific fertility circumstances.
Preparing for a consultation
You can prepare for a consultation by collecting your and your partner’s personal and family health histories. While it is helpful to have as much family history as possible, especially that of parents, siblings, aunts, uncles, and grandparents, we understand that some people do not have access to this information. That is okay—we can help you work with what you know to make the best possible decisions for your family.