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Testing

TYPE OF TEST:

TESTING PERIOD:

  • Before and during pregnancy

TESTING FOR:

RESULTS AVAILABLE:

  • Usually within 2 weeks

 




Fragile X Carrier Testing

Reasons for testing Testing for... How the test works What's not detected
Understanding results        

Reasons for Testing

Understanding your testing options helps you prepare for the future.

You may have many reasons for testing, including if you have:

  • a family history of fragile X syndrome
  • a family history of mental retardation, developmental delay or autism of unknown cause
  • infertility problems associated with elevated follicle stimulating hormone (FSH) levels, premature ovarian failure (POF) or unexplained early menopause

Fragile X syndrome is found among all ethnic backgrounds and racial groups. Approximately 1 in 260 women in the general population are carriers of the abnormal gene that causes fragile X syndrome. Additionally, women of all ages can have a child affected with fragile X syndrome, whether or not they have had previous healthy children.

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”.


Testing for...

Fragile X syndrome is caused by a change in the Fragile X Mental Retardation (FMR1) gene. This abnormal gene can be passed from generation to generation. Fragile X syndrome is usually passed on by a gene that is carried by a woman. Therefore, in the general population, women, and not men, are usually offered carrier testing.

“Fragile X carrier” is a term used to describe someone who has an abnormal FMR1 gene, but does not show any obvious symptoms of fragile X syndrome. Women who are fragile X carriers have up to a 50% chance (or a 1 in 2 chance) to have a child with fragile X syndrome. Men who are fragile X carriers pass the abnormal gene to all of their daughters but none of their sons. Daughters of carrier men are expected to be intellectually normal and are at risk to have affected children.

The genetics of fragile X syndrome are complicated. Genetic counseling is recommended when someone has a family history of fragile X syndrome or is shown to be a carrier of fragile X.


How the test works

The fragile X carrier test provides specific information about whether or not individuals are fragile X carriers, and, therefore, about their risks of having a child with fragile X syndrome. The test is performed on a small sample of blood and results are usually ready within two weeks. Testing provides accurate results more than 99% of the time. Other causes of mental retardation are not identified through this test.


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

There are four different results that you may have from the fragile X test: negative, intermediate, premutation and full mutation.

Test result What the test result means
for the mother
What the test result means
for the pregnancy
Negative You are not a carrier for the most common alteration in the FMR1 gene. Your baby is not at increased risk for fragile X syndrome.
 
Intermediate Your results fall in the range between negative and premutation. Your baby is not at increased risk for fragile X syndrome.
Future generations may be at risk for fragile X syndrome.
Genetic counseling is recommended.
Premutation You are a carrier for the altered FMR1 gene.
You may be at risk for early menopause.
Genetic counseling is recommended.
Your baby is at risk for fragile X syndrome.
Genetic counseling is recommended and prenatal diagnostic testing should be considered.
Full mutation You are a carrier for the altered FMR1 gene Your baby is at risk for fragile X syndrome.
Genetic counseling is recommended and prenatal diagnostic testing should be considered.

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 abnormal gene
  • 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