Carrier Screening
This informational page will cover what carrier screening is, what it can detect for, who it is for, and the limitations of the assessment.
What is Carrier Screening?
Carrier screening is a test that anyone can consider, whether they are planning for a pregnancy, are currently pregnant, or are planning to be a donor. It can be performed through either providing a blood or saliva sample.
The goal is to identify if a person or reproductive pair could have an increased chance to have a pregnancy with select genetic conditions. Carriers usually do not have symptoms of the condition or a family history, so they don’t know their status until testing is performed.
Anyone can consider carrier screening, but no one has to have it performed. It is often offered during a pregnant person’s first few prenatal appointments. The screen is most helpful before pregnancy though, so a person can have all the reproductive options available to them.
Who Can Consider Carrier Screening?
Carrier screening can inform a person or reproductive pair they are at an increased chance to have a pregnancy with select genetic conditions. With this knowledge, they can consider alternative reproductive options such as additional testing in pregnancy, testing embryos, utilizing donors, or plan to test their baby after birth. However, carrier screening is not a good fit for everyone. Not everyone feels empowered to know this information or may not want or be able to be faced with alternative pregnancy options. The test has limitations and people may not be comfortable with an amount of uncertainty from the test. It’s a personal decision of whether or not to have carrier screening.
Why Have Carrier Screening?
If a person or reproductive pair find that there is an increased chance to have a pregnancy with a genetic condition, then there are a testing options available should someone desire additional testing. These include the following, but this may not be a comprehensive list and not everyone has access to the below options:
test the pregnancy by having a CVS or amniocentesis
test the baby after birth
test embryos by undergoing in vitro fertilization (IVF)
consider donor egg, sperm, or embryo
consider adoption and/or foster care
consider being childfree
What reproductive options are available?
There are many conditions that can be tested through carrier screening. Some of the conditions may have treatment whereas others may not. Conditions can be life limiting or require someone to use adaptive technologies to learn and perform daily living tasks. There are many different conditions that can be tested. The conditions may have autosomal recessive or x-linked inheritance patterns.
What Kind of Conditions Are Tested?
Some of the common conditions that can be screened:
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Cystic fibrosis is a condition that impacts many parts of the body, typically the lungs and digestive system. The body produces sticky, thick mucus that makes it difficult to breathe, fight off infection, and absorb nutrients. Because of these symptoms, overall health can be impacted and possibly life span. Treatment is available, but there is not yet a cure. There are tons of support and informational resources such as The Cystic Fibrosis Foundation.
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There are different types of spinal muscular atrophy (SMA), all of which impact motor development to some degree which can include breathing, digestion, and movement. For type 1 SMA, babies can pass away around 2 years of life after experiencing limited movement, feeding tubes, and breathing assistance. There are new treatments that have helped to lengthen lifespan and improve symptoms. https://www.curesma.org/Cure SMA is a well-known resource for support and treatment discussion.
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Hemoglobinopathies is a term used to refer to multiple conditions that impact either the structure or amount of hemoglobin. Hemoglobin is a structure found inside of red blood cells whose function is to carry oxygen from our lungs to the rest of our bodies. If the hemoglobin structure has an abnormal shape or there are not enough of the structure in the red blood cell, then our body and organs do not get enough oxygen. The most known hemoglobinopathy is Sickle Cell Disease, which is also called hemoglobin SS disease. It causes the hemoglobin to be abnormally shaped. There are support organizations for Sickle Cell disease such as the Sickle Cell Society.
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Fragile X syndrome is thought as one of the most common inherited forms of autism. Many individuals with Fragile X have autism, developmental delays, and intellectual disability. Fragile X is most often a condition that impacts genetic males. Females are usually thought of as carriers because Fragile X has X-linked inheritance pattern. However, genetic females can also have Fragile X, but it is not as common as it is for males. National Fragile X Foundation provides more details on what it means to be a carrier of Fragile X and highlights individuals making a difference: https://fragilex.org/.
How do you do decide which conditions to have testing for?
This is actually a much more difficult question to answer than what meets the eye. In the past, there were only a few conditions that were able to be tested for and were the most impactful to the health of the pregnancy. Now, genetic testing labs are able to test for hundreds of conditions. Some of these conditions have what some people may think of as mild symptoms and should not be tested for because they are not as “severe” as others. Determining if a condition is severe or not is subjective, so it is difficult to know what conditions people want to be tested for. For some, they want to be tested for all the possible conditions. Other people would like to be tested for conditions that would otherwise prompt them to make different reproductive decisions. People might just want to be tested for some of the most common conditions out there or what has been historically performed, which is ancestry or ethnicity based screening.
What do professional guidelines say?
American College of Obstetrics and Gynecology (ACOG) Committee Opinion Number 691 recommends that people are offered the option of carrier screening. Carrier screening is not required, and people can decline the evaluation. For those that want this information, ACOG offers guidelines on which conditions to test for and how many.
ACOG recommends the following:
1.
Cystic Fibrosis and Spinal Muscular Atrophy carrier screening as well as a Complete Blood Count for every pregnant person
2.
To consider one of the below approaches to carrier screening:
Ethnicity Based Screening
Based on a person’s self-described ethnicity or ancestry, there are certain conditions recommended for that person. For example, if someone has Cajun ancestry, then Tay Sachs carrier screening can be considered. The reason why someone’s ancestry may matter is because research has found that certain ethnicities have a higher chance to be carriers of certain conditions than the general population. One reason is because individuals of that population have children with individuals in that same population because of religious or regional reasons. There are evolutionary reasons too. Individuals with African ancestry are more likely to be carriers of Sickle Cell Disease because being a carrier can provide resistance to malaria which is more prominent in Africa. Ethnicity based screening is slowly moving away as an option as expanded carrier screening becomes more commonplace.
Expanded Carrier Screening
Expanded carrier screening means to test for multiple conditions at once. Many labs have created predefined panels of conditions that often include conditions like Cystic Fibrosis and Sickle Cell disease. However, not every lab tests for the same conditions. For example, some labs may test for 288 conditions whereas another lab may test for 150. The type of conditions that are on the panel vary in term of what features they cause. Some conditions may be life limiting and others may be considered “mild” because with medication, the individual would otherwise have no indication of having the condition. The lab decides which conditions are on the panel and often have a list of what those conditions are on their websites. Some labs will let a provider and patient pick which conditions to test for because not everyone wants to learn information about all of these conditions.
The American College of Medical Genetics (ACMG) also has guidance on this question as well.
This organization no longer recommends basing what conditions to test for on someone’s ancestry. Instead they have developed a tiered system of what conditions to include on a carrier screening panel. There are 4 tiers, but the organization recommends all individuals are offered tier 3.
How often someone has the condition and the severity were both considered in the organization’s decision. For tier 3, conditions that have a carrier frequency is >1 in 200 were included. A carrier frequency of >1 in 200 means that at least one person in 200 people would be a carrier of the condition. For conditions with x-linked inheritance, the condition would need to occur in 1 person out of 40,000 people to be included. Labs now offer carrier screening panels that follow ACMG’s guidance.
Let’s say someone is interested in carrier screening - what else do they need to know?
The most important thing to know is that there are limitations to the test.
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Carrier screening is designed to find the reasons that someone may be a carrier for a particular condition; however, as the genetics field evolves, we may find additional reasons that someone is a carrier. The technology and ability to identify someone has a carrier may improve as well. Currently, if someone receives a negative result, then it means that their risk to be a carrier has been reduced but not eliminated completely.
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Our understanding of genes and variants change over time; therefore, the interpretation of results can also change. A result can go from negative to positive or positive to negative.
Sometimes, it is difficult to know how impactful a condition will be to a person. We may be able to say that the person could have certain symptoms but not know exactly how severe those symptoms will be.
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Sometimes, a person identified as a carrier for a particular condition can have symptoms. They may be recommended to have evaluations for symptoms. For example:
Gaucher disease carriers have an increased risk of developing Parkinson's Disease.
Duchenne Muscular Dystrophy carriers can have cardiac complications.
Ataxia-telangiesctasia carriers have a higher chance of developing breast cancer than the average person who isn’t a carrier
What would someone do if they were identified as a carrier?
1. Contact their health care provider.
The provider who ordered the test or a genetic counselor can help with next steps.
2. Learn about the condition(s)
Is the condition x-linked or autosomal recessive? If x-linked, then the genetic male partner/sperm donor may not need carrier screening. If autosomal recessive, then in order to know the chances to have a baby with the condition, the partner/donor will need to be tested.
3. Learn about the options moving forward
Some people do not want to do any additional testing after receiving their carrier screening results. Other folks may consider in-vitro fertilization (IVF) with preimplantation genetic testing (PGT), prenatal diagnosis, adoption, donor egg or sperm, etc.
Carrier screening specific blog posts
contact us at genetics@modernreproduction.org for any questions
Resources:
"Cystic Fibrosis", Accessed July 4 2022, https://medlineplus.gov/genetics/condition/cystic-fibrosis/.
Prior TW, et al., "Spinal Muscular Atrophy." GeneReviews, https://www.ncbi.nlm.nih.gov/books/NBK1352/
Hunter, Jessica, et al. "FMR1 Disorders." GeneReviews, https://www.ncbi.nlm.nih.gov/books/NBK1384/
Gregg, Anthony R et al. “Screening for autosomal recessive and X-linked conditions during pregnancy and preconception: a practice resource of the American College of Medical Genetics and Genomics (ACMG).” Genet Med vol. 23,10 (2021): 1793-1806. doi:10.1038/s41436-021-01203-z
Ghosh, Kanjaksha et al. “Guidelines for screening, diagnosis and management of hemoglobinopathies.” Indian journal of human genetics vol. 20,2 (2014): 101-19. doi:10.4103/0971-6866.142841
Committee Opinion No. 691: Carrier Screening for Genetic Conditions.” Obstetrics and gynecology vol. 129,3 (2017): e41-e55. doi:10.1097/AOG.0000000000001952