Spinal muscular atrophy (SMA) is a genetic condition that weakens muscles throughout the body. This makes it difficult to move, swallow, and in some cases breathe.
SMA is caused by a gene mutation that is passed from parents to children. If you’re pregnant and you or your partner has a family history of SMA, your doctor might encourage you to consider prenatal genetic testing.
Having genetic testing done during pregnancy can be stressful. Your doctor and a genetic counselor can help you understand your testing options so you can make decisions that are right for you.
If you’re pregnant, you might decide to get prenatal testing for SMA if:
- you or your partner has a family history of SMA
- you or your partner is a known carrier of the SMA gene
- early pregnancy screening tests show that your odds of having a baby with a genetic disease are higher than average
The decision about whether to get genetic testing is a personal one. You can decide not to get genetic testing done, even if SMA runs in your family.
If you decide to get prenatal genetic testing for SMA, the type of test will depend on the stage of your pregnancy.
Chorionic villus sampling (CVS) is a test that’s done between 10 and 13 weeks of pregnancy. If you get this test, a DNA sample will be taken from your placenta. The placenta is an organ that’s only present during pregnancy and provides the fetus with nutrients.
Amniocentesis is a test that’s done between 14 and 20 weeks of pregnancy. If you get this test, a DNA sample will be collected from the amniotic fluid in your uterus. Amniotic fluid is the liquid that surrounds the fetus.
After the DNA sample is collected, it will be tested in a laboratory to learn if the fetus has the gene for SMA. Since CVS is done earlier in pregnancy, you will get the results at an earlier stage of your pregnancy.
If the test results show that your child is likely to have the effects of SMA, your doctor can help you understand your options for moving forward. Some people decide to continue the pregnancy and explore treatment options, while others may decide to end the pregnancy.
If you decide to undergo CVS, your doctor may use one of two methods.
The first method is known as transabdominal CVS. In this approach, a healthcare provider inserts a thin needle into your abdomen to gather a sample from your placenta for testing. They may use a local anesthetic to reduce discomfort.
The other option is transcervical CVS. In this approach, a healthcare provider puts a thin tube through your vagina and cervix to reach your placenta. They use the tube to take a small sample from the placenta for testing.
If you decide to get testing done through amniocentesis, a healthcare provider will insert a long thin needle through your belly into the amniotic sac that surrounds the fetus. They will use this needle to draw a sample of amniotic fluid.
For both CVS and amniocentesis, ultrasound imaging is used throughout the procedure to help ensure that it’s done safely and accurately.
Getting either of these invasive prenatal tests for SMA may raise your risk of miscarriage. With CVS, there’s a 1 in 100 chance of miscarriage. With amniocentesis, the risk of miscarriage is less than 1 in 200.
It’s common to have some cramping or discomfort during the procedure and for a few hours after. You may want to have someone come with you and drive you home from the procedure.
Your healthcare team can help you decide if the risks of testing outweigh the potential benefits.
SMA is a recessive genetic disorder. This means that the condition only occurs in children who have two copies of the affected gene. The SMN1 gene codes for SMN protein. If both copies of this gene are defective, the child will have SMA. If only one copy is defective, the child will be a carrier, but will not develop the condition.
The SMN2 gene also codes for some SMN protein, but not as much of this protein as the body needs. People have more than one copy of the SMN2 gene, but not everyone has the same number of copies. More copies of healthy SMN2 gene correlates with less severe SMA, and fewer copies correlates with more severe SMA.
In almost all cases, children with SMA have inherited copies of the affected gene from both parents. In very rare cases, children with SMA have inherited one copy of the affected gene and have a spontaneous mutation in the other copy.
This means that if only one parent carries the gene for SMA, their child might carry the gene as well — but there’s very little chance of their child developing SMA.
If both partners carry the affected gene, there’s a:
- 25 percent chance that both of them will pass on the gene in a pregnancy
- 50 percent chance that only one of them will pass on the gene in a pregnancy
- 25 percent chance that neither of them will pass on the gene in a pregnancy
If you and your partner both carry the gene for SMA, a genetic counselor can help you understand your chances of passing it on.
SMA is classified based on the age of onset and severity of symptoms.
SMA type 0
This is the earliest onset and most severe type of SMA. It’s also sometimes called prenatal SMA.
In this type of SMA, decreased fetal movement is usually noticed during pregnancy. Babies born with SMA type 0 have severe muscle weakness and trouble breathing.
Babies with this type of SMA usually don’t live beyond 6 months of age.
SMA type 1
This is the most common type of SMA, according to the U.S. National Library of Medicine’s Genetic Home Reference. It’s also known as Werdnig-Hoffmann disease.
In babies who are born with SMA type 1, symptoms usually appear before 6 months of age. The symptoms include severe muscle weakness and in many cases challenges with breathing and swallowing.
SMA type 2
This type of SMA is usually diagnosed between the ages of 6 months and 2 years.
Children with SMA type 2 may be able to sit but not walk.
SMA type 3
This form of SMA is usually diagnosed between the ages of 3 and 18 years.
Some children with this type of SMA do learn to walk, but they may need a wheelchair as the disease progresses.
SMA type 4
This type of SMA isn’t very common.
It causes milder symptoms that typically don’t appear until adulthood. Common symptoms include tremors and muscle weakness.
People with this type of SMA often stay mobile for many years.
For all types of SMA, treatment generally involves a multidisciplinary approach with healthcare professionals who have specialized training. Treatment for babies with SMA may include supportive therapies to help with breathing, nutrition, and other needs.
The Food and Drug Administration (FDA) also recently approved two targeted therapies to treat SMA:
- Nusinersen (Spinraza) is approved for children and adults with SMA. In clinical trials, it’s been used in babies as young as
8 days old.
- Onasemnogene abeparvovec-xioi (Zolgensma) is a gene therapy that’s approved for use in infants with SMA
under 2 years old.
These treatments are new and research is ongoing, but they may change the long-term outlook for people born with SMA.
The decision about whether to get prenatal testing for SMA is personal, and for some it may be difficult. You can choose not to have testing done, if that’s what you prefer.
It may help to meet with a genetic counselor as you work through your decision on the testing process. A genetic counselor is an expert on genetic disease risk and testing.
It may also help to speak with a mental health counselor, who can provide you and your family with support during this time.
If you or your partner has a family history of SMA or you’re a known carrier of the gene for SMA, you may consider getting prenatal testing.
This can be an emotional process. A genetic counselor and other health professionals can help you learn about your options and make decisions that feel best for you.