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The Modern guide to genetic carrier screening

The Modern guide to genetic carrier screening

13 min read

Our understanding of genetics and inheritance has improved in a major way over the past several decades. While scientists originally thought that the traits of offspring were simply an average of the traits their biological parents had, we now know they’re passed from parents to offspring through genes that can interact with each other in complex ways. With genetic carrier screening, we can take a peek at some of our genes associated with certain, often serious or life-threatening, health conditions to better understand what our offsprings’ chances are of having those conditions.

In this article, we’ll go over the science behind genetic carrier screening, the different screening options, the conditions it can (and can’t!) tell you about, how it differs from other types of genetic tests (like those that tell you about your ancestry, or newborn screening), and the options you have after screening.

The bottom line: We recommend doing genetic carrier screening 6-12 months before you start trying to conceive. Testing earlier is also a good option (your genes don’t change over time!), but doing so closer to when you start trying to conceive ensures you’re benefiting from the most up-to-date genetic science and technology. Doing carrier screening before you've conceived also gives you more options than waiting until you're already pregnant would. It gives you more time to prepare, and to possibly take proactive steps so that you reduce the risk of having a child with a genetic disease.

Main takeaways

  • Genetic carrier screening can tell you and your partner whether you carry certain recessive genetic variants that you could pass on to biological children, and the probabilities of your children having certain genetic conditions.
  • Genetic carrier screening focuses on conditions that are caused by a known single gene. This, as well as guidelines set forth by medical societies, limits the conditions these tests can screen for.
  • Some screening options test for specific genetic variants based on your estimated ancestry, as the prevalence of disease-related variants can differ significantly among ancestry groups. Other options will test for the same wide range of disease-related variants for everyone, regardless of ancestry or known family history.
  • Genetic carrier screening can be done through your doctor or direct-to-consumer tests. The cost ultimately depends on many factors, like your health insurance and the number of variants screened.
  • Unlike some other types of screening in the US, genetic carrier screening is not mandatory. The American College of Obstetricians and Gynecologists (ACOG) recommends that all people trying to get pregnant or currently pregnant be given the option, but whether or not they choose to pursue genetic carrier screening is a personal decision.
  • People use carrier screening to help them sort through different options they have when it comes to deciding if and how they want to have kids. Doing carrier screening before you conceive gives you the most options.

What is genetic carrier screening?

Carrier screening is a type of genetic test, either conducted with a doctor or at home, that analyzes DNA via blood or saliva sample to assess people's carrier status for diseases caused by known genetic variants that lead to recessive conditions. (We cover the differences between doctor-provided and at-home tests, as well as blood versus saliva sample, a little later.)

Here’s what that means: At conception, we get two copies of chromosomes 1 through 22 (we’re leaving the X and Y chromosomes out for the time being) with the same sets of genes — one from the person who provided an egg, and one from the person who provided the sperm.

When someone has two copies of a disease-related variant, they’ll be affected by that disease. If someone only has one copy of the disease-related variant, they’re called a carrier because they “carry” the variant, but aren’t affected themselves.

What can genetic carrier screening screen for?

The diseases included in carrier screening tests are called monogenic autosomal recessive disorders. This loosely translates to disorders caused by a single gene on your first 22 chromosomes that only show up if you have two copies of the genetic variant. The monogenic disorders most commonly screened for are present in up to 1 in 200 births in the US, and are the primary (but not only!) contributors to infant mortality.

(There are a whole host of conditions that simply can’t be screened for, like diseases caused by many genes or diseases that don’t have a strong genetic component.)

If both biological parents are carriers of a disease-related variant, this doesn’t mean their offspring will definitely have the disease. There’s a 25% chance that any given offspring will have both copies of the variant (which will result in passing on the recessive disease), a 25% chance of having no copies of the variant, and a 50% chance of one variant. (Fun fact: Those good old Punnett squares from biology class were teaching you this very concept.) This is why it’s possible to be a carrier for a disease even if you have no known family history of it: Even when both biological parents are carriers (which is not always the case), there is only a 25% chance of having an offspring with the disease.

What are the benefits of genetic carrier screening?

Genetic carrier screening gives you information about the probabilities you’re dealing with when it comes to the health of your future offspring, which can then be used to make proactive decisions to mitigate their risks of having serious genetic conditions.

The range of decisions you have depends on if you do carrier screening pre- or post-conception, and the greatest potential benefits are when carrier screening is done before conception. It’s for this reason that ACOG recommends screening before conceiving, when possible. It's also worth noting here that doing carrier testing early can alleviate any stress that testing while pregnant (and waiting weeks for the test results) might bring.

If two opposite-sex reproductive partners learn they’re both carriers for the same condition and they haven’t conceived yet, they have the widest range of options if they want to have biological children:

  • They can try and conceive naturally using their own eggs + sperm, knowing there’s a 25% chance a child will have the condition. People who choose to go down this route and conceive can also do a genetic test on the fetus. If the fetus has two copies of the disease-related variant, the biological parents can make the decision to end the pregnancy or prepare for birthing and raising a child with a genetic condition.
  • They can conceive using their own eggs and sperm, but use preimplantation genetic testing (more on this in the next section!) with other assisted reproductive technologies like in-vitro fertilization (IVF) to reduce the risk of offspring having the condition.
  • They can use donor eggs or donor sperm without the same genetic variant.
  • They can adopt.

If genetic carrier screening is done after someone is pregnant, there are significantly fewer options available. Of the four options listed above, people who do carrier screening once already pregnant only have the first option available to them. If two people had conceived without fertility treatment and then found out they were both carriers for the same disease-related variant, they can make the decision to end the pregnancy, or prepare for the possibility of having a child with a genetic condition.

Who should do genetic carrier screening?

Unlike some other types of screening in the US (like newborn screening, which we’ll get to in a bit), genetic carrier screening is not mandatory. ACOG recommends that all people trying to get pregnant or currently pregnant be given the option, but whether or not they choose to pursue genetic carrier screening is a personal decision.

If two reproductive partners using their own eggs and sperm choose to undergo genetic carrier screening, typically one partner will be tested first. ACOG suggests that the partner who has higher odds of being a carrier of a variant (based on racial/ethnic background or known family history) be the one tested first.

Despite this suggestion, in the real world, the person with ovaries is often tested first. If the person tested first is not a carrier for anything, it isn’t necessary to test the second partner — after all, their offspring would need to inherit the same variant from each parent to have the disorder. If that first partner is found to be a carrier, doctors would then recommend the second partner getting screened. That being said, it’s totally possible for two partners to get tested simultaneously (and is recommended in certain cases, like if there are time constraints).

What about in cases where donor eggs and/or sperm are being used?
Donors will often be thoroughly screened — they’ll provide information about their known family’s medical history, and go through carrier screening themselves. The screening protocol may differ depending on the egg/sperm bank, but some places screen donors for up to 100 genetic conditions.

How long before conception should genetic carrier screening happen?

Dr. Sharon Briggs, Modern Fertility's head of clinical product has a PhD in genetics. She recommends that people consider genetic carrier screening 6-12 months before they start trying to conceive: “It’s a good balance of getting the most up-to-date technology while still being proactive,” she explains.

DNA sequencing technology is continually improving over time, as is our knowledge of disease-causing variants. Today’s carrier screenings are more comprehensive than carrier screenings of even a few years ago, and they will continue to improve. But because your genes don’t change over time, there's no need to redo genetic carrier screening if you've already one it — even if the technology has advanced since. One genetic carrier screening is valid for your whole life.

What are the different types of genetic carrier screening?

Carrier screening is done by analyzing DNA in either blood or saliva. There are no differences in terms of what you can learn from blood- and saliva-based tests; which one is collected just depends on which specimen type the lab is set up to analyze. It also depends on if you’re pursuing testing through your doctor or through direct-to-consumer, at-home tests (at-home tests are more likely to collect saliva than blood). Companies like Invitae and Sema4 offer at-home screening options (but Sema4 still needs to be ordered by your doctor first). The cost of pursuing genetic carrier screening, whether it be through your doctor or through a direct-to-consumer option, depends on your health insurance, and on which screening option you go with.

There are a couple of different options as far as what sort of screen you can get. If you pursue testing through your doctor rather than through a direct-to-consumer company, they may prefer one screening protocol over others; ultimately, you as the patient have the final say in dictating which screening approach you pursue.

Option 1: Basic screening

ACOG’s current recommendation for all people interested in genetic carrier screening, regardless of racial/ethnic background and known family history, is to screen for the following conditions:

  • Cystic fibrosis: A condition characterized by alterations in mucus production, that can lead to blockages, damage, and infections in the body
  • Spinal muscular atrophy: A condition in which motor neurons, which are the cells responsible for things like breathing, swallowing, and walking, are progressively destroyed
  • Hemoglobinopathies (blood disorders) like sickle cell disease (aka sickle cell anemia) and alpha- and beta-thalassemia: Conditions in this group are characterized by changes in the number or function of red blood cells, which are necessary to carry oxygen throughout the body

These conditions are all life-threatening — cystic fibrosis and spinal muscular atrophy in particular are the first and second most fatal recessive single-gene disorders — and carrier frequencies for these conditions are relatively high in the general population. Since these conditions are life-threatening and fairly common, ACOG recommends everyone screen for them. Exactly what'll be offered through testing with your doctor or at home, though, may differ.

Option 2: Targeted screening

Targeted screening takes self-reported information about your family history and race/ethnicity to determine what additional conditions should be screened for. The underlying logic is this: We know that certain conditions are more common in certain ethnic groups, and that for some conditions, you’re more likely to have it if your genetic relatives have it. Targeted screening uses this information to test only for the specific disease-associated variants that you may be likely to have.

Here are some examples of common targeted screens:

  • Tay-Sachs disease in people of Ashkenazi Jewish descent: While an estimated 1 in 30 people of Ashkenazi Jewish descent are a carrier for Tay-Sachs, this drops to about 1 in 300 in other populations.
  • Sickle cell disease in people of African descent: Recent data from the US suggests that while 3 in every 1,000 white infants are a carrier of the sickle cell variant at birth, 73 in every 1,000 infants of African descent are.
  • Fragile X syndrome in people with a known family history of it: Fragile X is the leading cause of developmental delays and disabilities. An estimated 1 in 250 people with ovaries are carriers of variants associated with Fragile X syndrome, and those with a family history of Fragile X syndrome are twice as likely to be carriers.

(Potentially worth keeping in mind here: Race in studies of disease and disease-related variant prevalence is typically based on participants’ self-report, rather than any sort of genetic tests of ancestry. These self-reports aren’t always accurate).

Targeted screening is a good option, but it may not be for everyone. Not everyone has complete info on their biological family’s medical history — and the concept of mutually exclusive “ethnic groups” has gotten increasingly blurry as the world becomes more connected, and populations that at one point in the past may have been genetically distinct mix together. This melting pot-ness is great for genetic diversity and for humanity, but undermines the validity of picking and choosing what variants to look for based on someone’s estimated “ethnicity.”

An important caveat here: Targeted screening can be somewhat flexible to what you want. If you’re pursuing carrier screening through a doctor, you can specifically ask to get screened for any condition, and they should accommodate this request. When it comes to at-home testing, some options may offer targeted screening for different populations (though the majority offer expanded carrier screening panels).

Option 3: Expanded screening

The expanded carrier screening approach is agnostic with respect to known family history and ethnicity, and tests all people for the same broad range of conditions. This approach is newer than targeted screening, and is in no small part due to incredible advances in the speed, ease, and cost of DNA sequencing technology. Though the exact number differs based on discussion with your doctor or the company offering the test, expanded carrier screenings can simultaneously test if someone is a carrier for 100+ conditions.

The pros and cons

We’re huge fans of having access to having information about our own bodies, and each screening type certainly provides us with valuable information. One potential drawback to expanded screening is that the more conditions you test for, the more likely you are to find that you are a carrier for something — even if it’s something that’s very rare (making it super unlikely that your reproductive partner would also be a carrier for the same thing).

Whatever option you choose, genetic counselors have your back. Their job is to provide clarity, education, and support to people as they learn about their genetic risk factors and diagnoses. Though speaking with a genetic counselor isn’t a requirement when doing carrier screening, it’s important to keep in mind that genetic counselors are trained in both medical genetics (some of whom further specialize in prenatal and infertility genetics!) and in counseling, making them valuable resources you can rely on. You can use this tool to help you find genetic counseling near you.

How does genetic carrier screening differ from other genetic tests?

Genetic carrier screening is a powerful tool, but it’s not a stand-in for other types of genetic tests. Genetic carrier screening gives you information about the probability your offspring will have certain conditions or be a carrier for them; it doesn’t tell you with certainty what variants they will have or whether they’ll inherit a genetic condition, but it does give you options to reduce the risk.

Preimplantation genetic testing for monogenic conditions (PGT-M) is designed to test an embryo for abnormalities and/or variants associated with monogenic diseases before it’s transferred to the uterus. With PGT, you don’t learn about the probability of your offspring having a certain genetic variant. By evaluating the genes of the fetus rather than only learning about the probability of certain genetic variants, you’ll get concrete data points about what variants that fetus has. Because PGT can not test for all recessive conditions, people who choose this need to know in advance what disease-related variants they want to test for (and that’s where doing carrier screening preconception comes in).

Noninvasive prenatal testing (NIPT) is often used to test whether a fetus has a normal number of each chromosome. Typically, a fetus should have two copies of each chromosome, and when there are greater or fewer copies of a given chromosome, this is called aneuploidy. Down syndrome, for example, is caused by having one extra copy of chromosome 21. NIPT is done by taking a blood sample from the pregnant person and looking for DNA from the fetus.

By sequencing the DNA of the fetus, NIPT can detect if the fetus may have more or fewer copies of each chromosome. It’s important to keep in mind that because NIPT isn’t directly looking at the full genes of a fetus, it’s possible to get false positives or false negatives (meaning, it’s possible for NIPT to conclude your fetus has an abnormality in its number of chromosomes when in reality it does not, or vice versa).

Newborn screening is the process of testing a newborn for life-threatening diseases soon after it’s born. While newborn screening is federally mandated in the US, states and territories can decide what conditions they screen for specifically (you can check what’s screened for where you live here). Newborn screening looks for signs that a baby has a condition — not whether they may carry or don’t carry variants for single-gene disorders. Genetic tests along with blood tests measuring a range of different hormones, enzymes, proteins, are used to detect rare developmental, metabolic, and genetic disorders that can be treated early in life.

Because genetic carrier screening, PGT, and newborn screening all test for different things and provide different information, none can be considered stand-ins for the others. If you’ve done a genetic carrier screen previously, there are still important and different things you can learn from NIPT and newborn screenings, and vice versa.

Bottom line

Genetic carrier screening can help you understand the chance that you pass on a genetic condition to your biological children. Genetic carrier screening done before conception gives you the widest range of reproductive options, and this can be pursued through your healthcare provider or through direct-to-consumer tests. Some people may find this information useful in guiding their reproductive decisions, and others may not want to pursue this information —-- the decision is totally up to you.

Our personal recommendation? As a company built on providing people with information about their own bodies, we’re all about understanding the genetic risk factors you and your reproductive partner may carry before you start trying to conceive. This knowledge can be used to make proactive decisions to mitigate these risks — or to just better understand and prepare for future possibilities.

This article was medically reviewed by Dr. Sharon Briggs, Modern Fertility's head of clinical product development.

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Talia Shirazi, PhD

Talia is a clinical product scientist at Modern Fertility. She's passionate about reproductive health + behavioral neuroendocrinology. Talia received her PhD in biological anthropology at Penn State.

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