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Carrier Screening for Spinal Muscular Atrophy

Updated on May 27, 2021
Medically reviewed by
Evelyn O. Berman, M.D.
Article written by
Sarah Winfrey

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder that causes muscle weakness and degeneration over time. This condition affects approximately 1 out of every 10,000 live births.

The American College of Obstetricians and Gynecologists recommends that pregnant women or those considering pregnancy undergo carrier screening for SMA. Carrier screening, or carrier testing, can determine whether you carry a gene mutation that contributes to certain genetic disorders. When performed before or during pregnancy, carrier screening can inform you of the chances that your baby will have a genetic disorder, including SMA.

Carrier screening for SMA can provide valuable information about your family’s genetics.

What Is an SMA Carrier?

SMA is a genetic disorder associated with a particular gene known as the survival motor neuron 1, or SMN1, gene. Genes, like chromosomes, are passed on in pairs, one from each parent. Most people have two functioning copies of the SMN1 gene. Having two nonfunctioning copies of this gene causes spinal muscular atrophy.

In relation to SMA, a “carrier” refers to a person who has inherited one nonfunctioning and one functioning copy of the SMN1 gene. Between 1 in 40 and 1 in 60 people are SMA carriers.

Carriers’ bodies can still use the functioning copies of this gene, so they don’t show the signs and symptoms of SMA. The carrier can still pass on either the functional or nonfunctional gene to a child.

A child must inherit two nonfunctioning SMN1 genes — usually one from each parent — to be diagnosed with SMA. This is because SMA is a recessive condition. If only one parent is a carrier, the child’s risk of having SMA is low, although they have a 50 percent chance of being a carrier. If both parents are carriers, there is a 25 percent chance their child will be diagnosed with the condition.

How Does Carrier Screening for SMA Work?

Carrier screening for SMA requires DNA testing, which entails a simple blood test. A medical practitioner will take a blood sample, then send it to a genetic-testing facility for analysis. In most cases, one partner is tested first. If they test positive as an SMA carrier, their partner is then tested.

Keep in mind that testing positive as an SMA carrier does not mean you have SMA or that you are likely to develop the condition. It only offers information about your chances of passing it on.

Is Carrier Screening for SMA Accurate?

A carrier-screening blood test can detect the mutation most commonly associated with SMA, with 90 percent accuracy. However, there are mutations associated with SMA that cannot yet be identified with modern screening technology.

In extremely rare cases, the SMN1 gene can undergo spontaneous changes during the production of an egg (ovum) or sperm cell. This can result in a child being affected by SMA, even if only one parent is a carrier. For this reason, a child who tests negative as a carrier may still develop SMA.

Race and ethnicity can affect the accuracy of a carrier screening. Studies have found DNA testing to be accurate in detecting 95 percent of carriers among most populations, but the rate is closer to 70 percent among Black people. This is because the incidence of a particular hard-to-detect mutation is higher in Black people than other individuals.

Keep in mind that carrier screening cannot tell you with 100 percent certainty a couple’s chances of giving birth to a child with SMA. Also, if screening shows that both partners are carriers, it cannot determine how severe a child’s SMA will be if they do inherit it.

What Is the Probability of a Child Having SMA or Being a Carrier?

Understanding how recessive SMA-causing genes are passed down can help you determine your chances of having a child who has SMA or is a carrier. If only one partner is a carrier, the child has a 50 percent chance of being a carrier, but their chance of having SMA is close to zero.

If both partners are SMA carriers, there are three possible outcomes:

  • In 25 percent of these cases, the baby will not be a carrier and will not have SMA.
  • In 50 percent of these cases, the baby will be born a carrier but will not have SMA.
  • In the remaining 25 percent of these cases, the baby will be born with SMA.

Who Should Consider Carrier Screening for SMA?

In the general population, about 1 in 50 people are SMA carriers. You can be a carrier regardless of your family’s medical history. Therefore, anyone may choose to undergo SMA-carrier screening, even if they don’t have symptoms.

However, carrier screening is especially advisable for specific groups.

Pregnant Women or Women Trying to Conceive

Women who are or are trying to become pregnant may undergo SMA carrier screening as part of a standard prenatal-screening package. During this screening, you can ask your health care provider to test for additional disorders as well.

It’s important to understand that you can choose whether or not to undergo carrier screening for SMA. Whether you decide to get tested is up to you — there is no right or wrong choice.

People With a Family History of SMA

Individuals with a family history of SMA or with family members who are carriers are more likely to be SMA carriers themselves. For this reason, they may choose to seek out testing or talk to a genetic counselor about their chances of being carriers.

What Should You Do if You’re a Carrier for SMA?

The test results of your SMA screening may be used in different ways, including as part of the family-planning process. Reproductive choices are sensitive and extremely personal. As Cure SMA notes, your family has the right to choose whatever option is best for you.

If you are found to be a carrier, the next step may be to have your partner tested.

If you or your partner are carriers and want to have children, you have several options, including adopting, undergoing prenatal testing, or having a preimplantation genetic diagnosis. This testing screens embryos for the presence of genetic disorders like SMA. Then, through in vitro fertilization, an unaffected embryo can be inseminated and transferred to a woman’s uterus.

If you are pregnant and would like to determine whether your child may have SMA, you can have a prenatal diagnosis as early as the 10th week of your pregnancy.

You may also speak to a genetic counselor as part of the carrier-screening process. These health care professionals are trained to help people at increased risk for or diagnosed with genetic conditions like SMA. Genetic counseling can help you understand the testing process and results and help you determine any next steps you may want to take.

Get Support

On mySMAteam, the social network for people with spinal muscular atrophy and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with SMA.

Have you undergone carrier screening for SMA? Share your experience in the comments below, or start a conversation by posting on your Activities page.

Evelyn O. Berman, M.D. is a neurology and pediatric specialist and treats disorders of the brain in children. Review provided by VeriMed Healthcare Network. Learn more about her here.
Sarah Winfrey is a writer at MyHealthTeams. Learn more about her here.

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