Prenatal Screening for Spinal Muscular Atrophy

Spinal muscular atrophy is a disease that most often strikes babies and young children, making it hard for them to move their muscles. SMA is passed down in families from parents to their children.

It's caused by changes, or mutations, in a gene called the survival motor neuron gene 1 (SMN1). Genes come in pairs, and we get one from each parent. They determine things like eye color, hair color, and the way your body works.

This SMN1 gene makes a protein that helps the nerves that control our muscles work as they should. When the gene is broken, it can lead to muscle weakness and movement problems.

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A child who has SMA has two broken copies of this gene -- one from their father and one from their mother.

Why Screen for SMA?

Somewhere between one in 40 and one in 60 people have a damaged gene that can cause SMA. If both parents have the gene, the chance that they’ll have a baby with SMA goes up.

If you have the gene, you won’t have any symptoms. So there’s no way of knowing you have it unless you get tested. If someone in your family has SMA, you have a higher chance of having the gene and passing it on to a baby.

How Does Screening Work?

If there’s a chance you might carry the SMA gene, your doctor could suggest tests. You can screen for the gene before you try to have a baby, or wait to have the screening until you become pregnant.

Carrier screening. Before you decide to have a baby, you might choose to get a simple blood test to see if you carry the SMA gene. One parent is usually tested first. If they have it, then the other parent is tested. If only one of you has the SMA gene, the chance of having a baby with the disease is very low.

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If both you and your partner are carriers, you might want to talk to a genetic counselor about other ways to be parents. For example, donated eggs or sperm, or adopting a child.

If you choose to get pregnant with your partner, your doctor will likely suggest one of the following tests.

Chorionic villus sampling (CVS). This can be done between the 10th and 14th weeks of your pregnancy. Your doctor will use ultrasound to help remove a tiny piece of tissue from the placenta.

The tissue includes small structures called chorionic villi that have fetal DNA. This DNA will be tested for SMA.

Amniocentesis. You can have this test 16 to 20 weeks into your pregnancy. Using an ultrasound for guidance, your doctor will insert a small needle into your abdomen. The needle removes a tiny amount of amniotic fluid that has fetal DNA. The fluid is then tested for SMA.

Next Steps

If tests show that your baby is likely to have SMA, talk to your doctor or a genetic counselor to see what your options are, including treatments.

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