Reasons for testing
Understanding your testing options helps you prepare for the future.
You may have many reasons for testing, including if you have:
- concerns regarding inherited diseases
- an increased risk of being a carrier of an inherited disease due to ethnic background
- a family history of a specific inherited disease
Genetic disorders are caused by a change in a gene; this change is called a mutation. Every person has two copies of each gene, one inherited from each parent. A carrier is a person who has one normal copy of a gene and one abnormal copy. Having one normal gene is enough to prevent the disease. However, if both parents are carriers of the same abnormal gene, there is a chance that each parent will pass his or her abnormal gene on to their baby. If the baby inherits two copies of the abnormal gene, the baby will have the disease. Diseases that are inherited this way are called autosomal recessive diseases.
Couples may decide to have carrier testing to find out if they are carriers, and therefore are at risk of having a baby with one of these genetic diseases. If both parents are carriers of the same disease gene, prenatal diagnosis can be performed to determine whether or not the fetus is affected.
Given the severity and frequency of Spinal Muscular Atrophy (SMA), the American College of Medical Genetics (ACMG) recommends SMA carrier screening be offered before conception or early in pregnancy to everyone. The primary goal is to allow carriers to make informed reproductive choices.
If both parents are carriers of an abnormal gene, there is a chance that each parent will pass the abnormal gene on to their child. An individual with two abnormal genes for an autosomal recessive disease will be affected with the disease. When both parents are carriers, with each pregnancy there is a:
- 25% (1 out of 4) chance that the child will inherit two normal genes, and will be neither a carrier, nor affected with the disease
- 50% (2 out of 4) chance that the child will inherit one normal and one abnormal gene, and will be a carrier of the disease (but not affected with the disease)
- 25% (1 out of 4) chance that the child will inherit two abnormal genes, one from each parent, and will be affected with the disease
You could be a carrier of Spinal Muscular Atrophy (SMA) even if no one in your family has SMA and even if you already have children without SMA. Carriers of the abnormal gene for SMA have no symptoms of the disease.
As with most inherited diseases, the risk for being an SMA carrier varies by ethnic background. Because SMA is such a severe disease with a high carrier frequency, ACMG guidelines recommend all people be tested regardless of race or ethnicity.
SMA Carrier Risk in People with No Family History of SMA:
- Caucasian - 1 in 47
- Asian Indian - 1 in 52
- Asian - 1 in 59
- Ashkenazi Jewish - 1 in 67
- African American - 1 in 72
- Hispanic - 1 in 68
If a relative of yours has SMA, or is known to be a carrier of SMA, your chance of being a carrier is greater based upon your family history.
If you are not sure whether you should have carrier testing, please talk with your doctor, a genetic counselor or other specialist in genetics. See “Questions to Ask Your Doctor”.
The purpose of SMA carrier testing is to see if a couple is at increased risk for giving birth to a child who will be affected with SMA. SMA carrier testing is done on a sample of blood. If testing determines that a couple is at high risk, prenatal testing (chorionic villi sampling [CVS] or amniocentesis) can be done to see whether or not the baby has inherited two abnormal SMA genes.
If the baby is determined to be affected with SMA, additional testing will be performed to help determine which type of SMA the baby may have.
How the test works
Carrier testing is performed by testing a blood sample to see if a person has one abnormal copy of a specific gene.
For a couple planning a pregnancy, one partner is usually tested first. If that person is found to be an SMA carrier, the other partner should be tested. If a woman is already pregnant, both partners may be tested at the same time.
What's not detected
Carrier testing usually tests for the most common mutations that can result in an abnormal gene. Carrier testing often does not detect less common mutations.
A negative result significantly lowers, but does not completely eliminate, the risk of being a carrier of SMA. Carrier screening does not detect less common abnormalities (mutations) that cause SMA.
For SMA, if the test determines that you are a carrier, the next step is for your partner to have carrier testing performed. Both parents must be carriers for the baby to be at risk for an autosomal recessive disease. If you are determined to be a carrier and your partner has a negative test result and no family history of SMA, the chance that your baby will have SMA is less than 1%. If testing determines that a couple is at high risk, prenatal testing (chorionic villi sampling [CVS] or amniocentesis) during pregnancy can be done to see whether or not the baby has inherited the disease. A follow-up visit with a perinatologist and/or genetic counselor may be recommended.
It is important to remember that if you and your partner are both found to be carriers of an abnormal SMA gene and have a child together, there is a 1-in-4 (25%) chance with each pregnancy that the child will be affected with SMA. This is true even if you already have other children with or without SMA.
If carrier testing shows both parents are carriers, you may be referred to a genetic counselor. There are several choices couples in your situation can make when thinking about possible future pregnancies. Some couples decide to:
- have prenatal testing (amniocentesis or CVS) to determine whether or not the baby has inherited the two abnormal genes
- accept this level of risk and have children without further testing
- go through in vitro fertilization and test the embryos using preimplantation genetic screening (PGS)
- adopt children
- use donor sperm or donor eggs
- not have children