It is difficult to identify fragile X syndrome (FXS) based on signs or symptoms alone. The signs of FXS can differ significantly from child to child, and symptom severity may range from insignificant to profound. If parents suspect their child has FXS, they should report all signs or symptoms to their child’s pediatrician. A physician will likely compile a medical history and may perform a physical examination of the child. However, the only way to determine the presence of FXS is through a DNA analysis. This may also be used to identify asymptomatic (without symptoms) carriers (both male and female).   

DNA analysis has been available since 1991. It can accurately and reliably identify the gene mutation responsible for FXS in children and adults. The analysis can be performed with a small sample of blood, skin, bone or tissue.

Prenatal DNA testing may also be performed. This can identify whether a fetus has inherited a full mutation of the gene, but cannot always identify whether the child will be born with mental retardation. Prenatal DNA tests can be performed in the following ways:

• Amniocentesis. A hollow needle is inserted into a pregnant woman’s abdomen, and a sample of amniotic fluid from around the developing fetus is removed and analyzed.
  
  
• Chorionic villus sampling. Vaginal removal of a small sample of the placenta (protective tissue that nourishes a fetus) for analysis.

DNA analysis has replaced previous FXS diagnostic tests, such as karyotyping (looking for abnormalities by examining images of matched pairs of chromosomes). The chromosomal analyses are not sensitive enough to detect mutations in the gene responsible for FXS. Thus, DNA analysis is preferred for its reliability, accuracy and ability to identify asymptomatic carriers as well as people with the full-blown syndrome.