Fragile X Carrier Screening
Carrier screening for Fragile X syndrome
What is this test?
Fragile X syndrome is the most common inherited cause of intellectual disability and a significant cause of autism spectrum disorder. It is caused by an expansion of CGG repeats in the FMR1 gene on the X chromosome. Women can silently carry a 'premutation' (55-200 repeats) that may expand to a 'full mutation' (>200 repeats) when passed to their children, causing the syndrome. This test determines your FMR1 repeat count, identifying carriers and those at risk of having affected children.
Key Benefits
Premutation carriers are typically healthy women who may not know they carry an expanded FMR1 gene that could produce affected sons.
Identifying premutation status before pregnancy allows couples to consider PGT-M, prenatal diagnosis, or other reproductive options.
FMR1 premutations are associated with premature ovarian insufficiency affecting up to 20% of carriers — early detection can inform fertility planning.
7-day turnaround provides timely results for family planning decisions.
How It Works
A simple blood sample is collected from the individual being tested.
DNA is isolated from the blood sample for molecular analysis.
Specialized PCR designed for repetitive sequences amplifies the CGG repeat region of the FMR1 gene.
Fragment analysis on capillary electrophoresis precisely determines the number of CGG repeats, distinguishing normal, intermediate, premutation, and full mutation ranges.
Results report the CGG repeat count and classification. Genetic counseling is recommended for premutation and full mutation carriers to discuss reproductive implications and personal health considerations.
Who should consider this test?
- All women planning pregnancy (ACOG recommends offering to all women)
- Women with a family history of intellectual disability, developmental delay, or autism
- Women with premature ovarian insufficiency (POI) — which is associated with FMR1 premutations
- Women with family members known to carry Fragile X premutation or full mutation
- Men with tremor/ataxia syndrome or family history of Fragile X
Conditions Screened
Accuracy & Clinical Evidence
Triplet-repeat PCR with capillary electrophoresis accurately sizes CGG repeat expansions from normal range (<45 repeats) through premutation (55-200) and full mutation (>200) with high reliability.
ACOG and ACMG recommend that Fragile X carrier screening be offered to all women considering pregnancy, particularly those with a family history of intellectual disability, autism, or premature ovarian insufficiency. Carrier frequency in the general female population is approximately 1 in 250 for premutations.
For Healthcare Providers
FMR1 CGG repeat analysis is performed using triplet-repeat primed PCR (TP-PCR) followed by capillary electrophoresis fragment sizing. The assay accurately sizes alleles in the normal (<45), intermediate (45-54), premutation (55-200), and full mutation (>200) ranges. Female samples with apparent homozygosity for normal alleles undergo additional analysis to rule out a masked expanded allele. AGG interruption analysis is available upon request to refine expansion risk estimation for premutation carriers.
Important Limitations
- FMR1 premutations are unstable across generations — the exact expansion risk depends on repeat length, AGG interruptions, and maternal age.
- Full mutations with methylation mosaicism may have variable clinical presentation.
- Male carriers of premutations do not expand to full mutations when transmitted to daughters but all daughters will be premutation carriers.
- This test screens for FMR1 repeat expansions only — it does not detect point mutations or deletions in FMR1 (rare causes of Fragile X syndrome).
Frequently Asked Questions
Test Details
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