How do health care providers diagnose Fragile X syndrome?

Health care providers often use a blood sample to diagnose Fragile X. The health care provider will take a sample of blood and will send it to a laboratory, which will determine what form of the FMR1 gene is present.1

Prenatal Testing (During Pregnancy)

Pregnant women who have an FMR1 premutation or full mutation may pass that mutated gene on to their children. A prenatal test allows health care providers to detect the mutated gene in the developing fetus. This important information helps families and providers to prepare for Fragile X syndrome and to intervene as early as possible.

Possible types of prenatal tests include:

  • Amniocentesis (pronounced am-nee-oh-sen-TEE-sis). A health care provider takes a sample of amniotic (pronounced am-nee-OT-ik) fluid, which is then tested for the FMR1 mutation.
  • Chorionic villus (pronounced KOHR-ee-on-ik VILL-uhs) sampling. A health care provider takes a sample of cells from the placenta, which is then tested for the FMR1 mutation.1

Because prenatal testing involves some risk to the mother and fetus, if you or a family member is considering prenatal testing for Fragile X, discuss all the risks and benefits with your health care provider.

Prenatal testing is not very common, and many parents do not know they carry the mutation. Therefore, parents usually start to notice symptoms in their children when they are infants or toddlers. The average age at diagnosis is 36 months for boys and 42 months for girls.2

Diagnosis of Children

Many parents first notice symptoms of delayed development in their infants or toddlers. These symptoms may include delays in speech and language skills, social and emotional difficulties, and being sensitive to certain sensations. Children may also be delayed in or have problems with motor skills such as learning to walk.

A health care provider can perform developmental screening to determine the nature of delays in a child. If a health care provider suspects the child has Fragile X syndrome, he/she can refer parents to a clinical geneticist, who can perform a genetic test for Fragile X syndrome.2


  1. National Fragile X Foundation. (2012). Testing. Retrieved June 7, 2012, from External Web Site Policy
  2. Bailey, D. B., Raspa, M., Bishop, E., & Holiday, D. (2009). No change in the age of diagnosis for fragile x syndrome: findings from a national parent survey. Pediatrics, 124, 527–533.

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