A Short Guide to Spinal Muscular Atrophy


In the past, the American College of Obstetrics and Gynecology (ACOG) recommended testing prospective parents for spinal muscular atrophy (SMA) only if their ethnic group showed an increased risk for children inheriting the condition. However, ACOG has now updated its guidelines, recommending screening for all women. What is SMA, and why should you get the test?

What Is Spinal Muscular Atrophy?
One of the most common genetic disorders, SMA affects 1 in about every 11,000 babies. SMA is a gene mutation, or mixed-up gene information, on the gene that produces proteins necessary for the nerve cells in the spinal cord that control your muscles. Since babies receive two copies of the affected gene – one from each parent – both parents must provide a faulty copy of the gene in order for the baby to inherit SMA. Parents and children with only one copy of the faulty gene are only carriers.

Spinal cord nerves deprived of the protein affected will die prematurely, leaving the muscles they control drastically weakened. Individuals with SMA experience muscle weakness that may interfere with many activities of daily life, including walking, standing, eating and even breathing. SMA occurs in 4 different types dependent on the age of onset:

  • Type I. Present at birth, Type I SMA is the most common as well as the most severe. Most babies born with Type I SMA die by age 2.
  • Type II. Type II SMA is present in childhood; children with this type of SMA have shorter lifespans than typical children and are usually unable to walk or stand without assistance.
  • Type III. This type of SMA does not appear until later in childhood and is less severe than Types I and II. Children with this type may be able to walk and stand on their own.
  • Type IV. This type is adult onset and is typically less severe.

Though mental capabilities are not affected, an individual with SMA will experience weakened muscles throughout their lifespan, often growing weaker over time. There is no cure for SMA, though recently approved therapies hope to improve the gene’s capabilities to make protein.

Why Should Women Get Tested for SMA?

As many as 1 in every 50 Americans are carriers for SMA, meaning they have inherited one copy of the faulty gene responsible for SMA without showing any symptoms themselves. Since two carriers hold a 1 in 4 chance of having a baby with the disorder, determining if parents are carriers can be crucial in early identification of SMA. If you undergo carrier screening for the disorder and find you are a carrier, your doctor can offer your partner testing as well as genetic counseling in the event both of you are carriers.

ACOG now recommends doctors discuss carrier screening with all women, while focusing on women in at-risk ethnic populations and women with a family history of genetic disorders. Common tests screen for multiple disorders, giving you the chance to make the decisions about your current or future pregnancy that are best for you. Ask your doctor which carrier screening may be right for your situation.


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