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DNA testing for Fragile X, developed in 1992, is accurate and can detect carriers and fully-affected individuals. It can be ordered by any physician; the blood sample is then sent to a lab that offers the test. It usually takes several weeks to get results.
Because symptoms of Fragile X can be subtle, especially in young children, and because it is so frequent in the general population (1 in 4000 males and 1 in 6000 females), Fragile X testing should be considered for any individual with signs of autism or unexplained developmental delay or intellectual disabilities.
Most major U.S. medical centers offer the Fragile X test. It may cost several hundred dollars but is usually covered by health insurance. Prenatal testing with chorionic villus sampling or amniocentesis allows diagnosis of FMR1 mutation while the fetus is in utero and appears to be reliable.
For more information about testing, talk to your doctor or genetic counselor. You can also browse this map of U.S. Fragile X Clinics
Since the 1990s, more sensitive molecular techniques have been used to determine carrier status. The fragile X abnormality is now directly determined by analysis of the number of CGG repeats using polymerase chain reaction (PCR) and methylation status using Southern blot analysis. By determining the number of CGG repeats on the X chromosome, this method allows for more accurate assessment of risk for premutation carriers in terms of their own risk of fragile X associated syndromes, as well as their risk of having affected children. Because this method only tests for expansion of the CGG repeat, individuals with FXS due to missense mutations or deletions involving FMR1 will not be diagnosed using this test and should therefore undergo sequencing of the FMR1 gene if there is clinical suspicion of FXS.
Source: wikipedia link
When a woman is a carrier of Fragile X, her chances of passing the mutation on to her children depend on the length of her own premutation. This article was written by experts in Fragile X syndrome and prenatal diagnosis.
Expansion of the Fragile X CGG Repeat in Females with Premutation or Intermediate Alleles
Am J Hum Genet. Feb 2003; by Sarah L. Nolin,1 W. Ted Brown,1 Anne Glicksman,1 George E. Houck, Jr.,1 Alice D. Gargano,1 Amy Sullivan,2 Valérie Biancalana,3 Karen Bröndum-Nielsen,4 Helle Hjalgrim,4 Elke Holinski-Feder,5 Frank Kooy,6 John Longshore,7 James Macpherson,8 Jean-Louis Mandel,3 Gert Matthijs,9 Francois Rousseau,10 Peter Steinbach,11 Marja-Leena Väisänen,12Harriet von Koskull,13 and Stephanie L. Sherman2