Reasons for testing

Understanding your testing options helps you to 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.

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

If you are not sure whether you should have carrier testing, please talk with your doctor, a genetic counselor or other specialist in genetics. In particular if you have a family history of a genetic disorder, or you or one of your relatives is known to be a carrier, your chance to be a carrier is higher than the rest of the population. See “Questions to Ask Your Doctor”.

Testing for...

Carrier testing determines if a couple is at increased risk of having a baby with a specific inherited disease, such as Tay-Sachs disease or cystic fibrosis.

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 a 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.

Understanding results

A negative result significantly lowers, but does not completely eliminate, the risk of being a carrier.

For autosomal recessive diseases, 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 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.

X-linked diseases are inherited through the mother. If the test determines that the mother is a carrier, 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.

If carrier testing shows that you are at risk to have a baby with a genetic disease, 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