What is Fragile X Syndrome?

Fragile X syndrome is the most common cause of inherited intellectual disability, affecting approximately one in 3,600 males and one in 4,000-6,000 females. It can occur in any ethnic group. Intellectual disability may range from learning disabilities to severe mental retardation and autism. Physical characteristics are often more pronounced in adult males and may include macrocephaly, enlarged ears, and long face with a prominent chin. Approximately 50% of females affected with fragile X syndrome have borderline or mild intellectual disability.

How is Fragile X Syndrome Inherited?

Fragile X syndrome is inherited in an X-linked manner because it is carried on the X chromosome. Women who are carriers can run a risk as high as 50% to have children with the disorder, depending on their specific carrier screen results. Women who are carriers also have an increased risk to develop early ovarian failure. Men (and sometimes women) who are carriers can develop fragile X-associated tremor/ataxia syndrome (FXTAS), an adult onset neurological condition. Genetic counseling is recommended for those found to be carriers of fragile X syndrome in order to discuss individual risk to have an affected child, as well as any personal implications.

Who Should be Offered Fragile X Carrier Testing?

Fragile X testing may be offered for women with a family history of:

  • Fragile X-related disorders
  • Unexplained mental retardation or developmental delay
  • Autism
  • Premature ovarian insufficiency (or elevated FSH <40 yrs)

Women who request fragile X carrier screening, regardless of family history, should be offered screening, after genetic counseling on the risks, benefits, and limitations of screening. Approximately one in 259 women in the general population are carriers of fragile X syndrome.

Is There Prenatal Diagnosis for Fragile X Syndrome?

Yes. Prenatal testing by chorionic villi sampling or amniocentesis can be performed to determine whether or not a fetus is affected with fragile X syndrome.


  1. FRAXA Research Foundation. Accessed September 2019