Fragile X syndrome: Prenatal screening and diagnosis

INTRODUCTION — Fragile X syndrome is an X-linked dominant disorder associated with a wide spectrum of clinical features, including intellectual disability, neuropsychiatric manifestations, autism spectrum disorder, and seizures. It is the most common inherited cause of intellectual disability. The clinical features vary depending upon mutation state (full mutation versus premutation), sex (males with a full mutation are more likely to have intellectual disability than females), degree of methylation of the fragile X messenger ribonucleoprotein 1 gene (hypermethylation worsens outcome), and variation of gene expression across tissues due to mosaicism, as shown in the table (table 1).

This topic will discuss preconception/prenatal screening and prenatal diagnosis for fragile X syndrome. The epidemiology, pathogenesis, clinical features, postnatal diagnosis, and postnatal management of the disorder are reviewed separately:

●(See "Fragile X syndrome: Clinical features and diagnosis in children and adolescents".)

●(See "Fragile X syndrome: Management in children and adolescents".)

PATHOPHYSIOLOGY

FMR1 gene and protein — The fragile X messenger ribonucleoprotein 1 gene (FMR1) is located on the X chromosome at Xq27.3 and most commonly has approximately 30 cytosine-guanine-guanine (CGG) trinucleotide repeats, with a "normal" range of approximately 5 to 44 CGG repeats. FMR1 produces the fragile X messenger ribonucleoprotein (FMRP).

As long as FMR1 has ≤44 CGG repeats, gene transcription produces an adequate level of FMRP (ie, a level not associated with adverse phenotypic effects) and this region of the X chromosome remains stable, passing from generation to generation without significant alteration.

Fragile X syndrome — Fragile X syndrome is primarily caused by expansion in the number of CGG repeats within FMR1; rarely, other pathogenic variants are detected within the clinical spectrum, including splice site, missense, deletion, nonsense, and frame shift variants [1]. Deletions or point mutations within FMR1 account for only 1 percent of cases [2].

As repeat size increases, stability decreases, thereby facilitating further increases in the number of repeats in the FMR1 region during gametogenesis (discussed below). The lower and upper repeat length boundaries are generally described as:

●Normal – 5 to 44 CGG repeats

●Intermediate expansion – 45 to 54 CGG repeats

●Premutation – 55 to 200 CGG repeats

●Full mutation – >200 CGG repeats

Expansion of CGG repeats within the premutation range (55 to 200 CGG repeats) increases the quantity of mRNA with toxic effects [3]. Expansion of CGG repeats to a full mutation (>200 CGG repeats) promotes hypermethylation of FMR1, resulting in impaired transcription and reduced production of FMRP, which may adversely impact prenatal and postnatal brain development and postnatal ovarian function.

The appropriate distinction between intermediate and premutation length is uncertain, but clinically relevant because intermediate expansion does not have a clinical phenotype, is not associated with the phenotype of premutation carriers, and does not expand directly to a full mutation in gametes, whereas females (but not males) with a premutation are at risk of expansion to a full mutation in gametes [4].

There is no cure for fragile X syndrome. Active areas of therapeutic research include manipulating the excessive methylation and replacing the deficient levels of FMRP [5,6]. However, symptoms can be managed by individualized education programs, speech and language therapy, occupational therapy, behavioral therapy, and pharmacotherapy. (See "Fragile X syndrome: Management in children and adolescents".)

FMR1 expansion during gametogenesis — The risk for FMR1 expansion in gametes depends on the sex of the parent, repeat size, frequency of adenine-guanine-guanine (AGG) trinucleotide interspersion, and pedigree. Postzygotic mitotic CGG length changes have also been reported; discordance for CGG repeat number between identical twins is assumed to be due to this mechanism [7].

Oocytes — Progressive generational expansion of CGG repeats occurs if two X chromosomes are present, as typically noted in females (46,XX). Expansion in the number of CGG repeats within the FMR1 gene may be as few as one repeat or over 100 repeats. Expansion during oogenesis is supported by the finding of full FMR1 expansions in the ovaries of premutation carriers. Whether this occurs during meiosis 1 or 2 remains unresolved [8].

The following variables affect the frequency of expansion to a full mutation:

●Number of CGG repeats – Among females with a premutation >90 CGG repeats, the likelihood of further expansion to a full mutation (ie, over 200 repeats) during oogenesis is at least 90 percent (table 2) [9,10]. The frequency is much less with fewer repeats.

Conservatively, the upper limit of the intermediate range may be considered 54 repeats, where 55 is a "potential" premutation and 56 to 200 is a premutation. In one study, females with 45 to 54 repeats had a 6.6 percent risk of expansion during oogenesis, but none expanded to a full mutation in one generation [11]. Reports of expansion to a full mutation during oogenesis among females with 55 to 59 CGG repeats are very rare (one case at 56 CTG repeats and two cases at 59 CTG repeats). When assessed by a cohort study and not case reports, the risk of the smallest premutations (55 to 59 CGG repeats) expanding to full mutations is <1 percent. (See 'Post-test counseling' below.)

●Pedigree – The frequency of expansion to a full mutation is higher when a full mutation exists in the individual's pedigree [10]. The frequency is lower when the pedigree does not exhibit individuals with known fragile X syndrome, developmental delay, or autism spectrum disorder.

●AGG trinucleotide interspersion – The frequency of CGG expansion to a full mutation is higher in individuals with a low frequency of AGG trinucleotide interspersion in the variable region. Because AGG acts as a stabilizer for the region, fewer AGG interspersions allows destabilization of the region and increases the likelihood of expansion [12-16]. In one study, 53 percent of full mutation expansions occurred from maternal alleles with no AGG interruptions, 43 percent occurred from maternal alleles with one AGG, and only 4 percent occurred from maternal alleles with two AGGs [13].

Contraction from a premutation to a normal allele is rare, but has been reported in mother-to-female offspring transmission. The role of AGG regions, which mitigate the expansion risk of the FMR1, do not appear to affect contraction rates [13].

Sperm — In males, sperm cells only carry premutation alleles, even when the male has a full mutation. In contrast to oocytes, expansion of a premutation to a full mutation does not occur in sperm [17]. Males with a premutation ("transmitting males") pass sperm with a premutation to all of their female offspring (who always inherit their father's X chromosome) and to none of their male offspring (who never inherit their father's X chromosome). In contrast to mother-to-female offspring transmission where contraction of the premutation is rare, one-third of female offspring who inherit a premutation from their father have contraction in the size of the expansion.

The molecular processes behind contraction of CGG repeats within the FMR1 gene remain unclear. Strand mismatch and DNA polymerase slippage are considered two likely contenders, and other molecular changes may play a role. The timing of these changes remains unknown, but evidence suggests the contraction can occur during both meiosis and mitosis [8,18-20].

Factors affecting phenotype — The fragile X syndrome phenotype depends on several factors, including the individual's sex, the number of CGG repeats, degree of FMR1 methylation, and variation of gene expression across tissues due to mosaicism (either in repeat size or methylation), as shown in the table (table 1). These factors play a role in the production of FMRP and lower protein levels result in a more severe phenotype.

The individual's sex chromosome composition (46,XY; 46,XX or variations) is an important factor because males have only one X chromosome and thus have the expanded CGG region in each of their cells. Females have two X chromosomes, one of which is normally silenced such that for any cell, the expanded CGG region is transcriptionally active from only one of their two X chromosomes. Since one of their X chromosomes is randomly inactivated, the overall number and distribution of cells with an expanded region is variable in females and results in a wide range of effects (from no discernable effects to classic fragile X syndrome characteristics). There is no accurate way to predict the phenotype in females with a full mutation.

Premutations are now recognized as the physiologic basis for fragile-X premutation-associated conditions (FXPAC). At the premutation level of expansion, the increased mRNA is thought to be the primary toxic agent with mitochondrial dysfunction and neuronal death consistent across the FXPACs. The symptoms are wide ranging but generally fall into one of the following categories: fragile X-associated tremor/ataxia syndrome (FXTAS), fragile X-associated primary ovarian insufficiency (FXPOI), and fragile X-associated neuropsychiatric disorders (FXAND) [21].

FMR1 expansion mosaicism (cells from the same patient have different numbers of CGG repeats and different methylation profiles) is common, resulting from the instability of the FMR1 region and postzygotic removal of part of the expansion [22,23]. For example, almost half of individuals with fragile X syndrome possess some cells with full FMR1 mutation regions and other cells with premutation regions.

A detailed description of the drivers of phenotype and the spectrum of clinical features of fragile X syndrome is available separately. (See "Fragile X syndrome: Clinical features and diagnosis in children and adolescents".)

PRECONCEPTION AND PRENATAL SCREENING

Rationale — Preconception or prenatal screening for fragile X premutations and full mutations is performed to identify individuals at risk for conceiving a child with clinical manifestations of fragile X syndrome. The prevalence of FMR1 premutation ranges from 1 in 250 to 813 male individuals to 1 in 110 to 270 female individuals, reflecting variation among populations globally [24]. When this information is available before pregnancy, carriers have an opportunity to seek interventions for avoiding pregnancy with an affected fetus, if desired. (See 'Preconception reproductive options' below.)

After pregnancy is established, carriers may choose to undergo prenatal diagnosis for fetal fragile X premutations and full mutations so they can prepare for the birth of an affected child or terminate a pregnancy with an affected fetus. (See 'Prenatal (fetal) diagnosis' below.)

Screening

●Targeted approach – A careful assessment of individual and family history is performed to identify persons with sufficient likelihood of premutation or full mutation carrier status to warrant screening. We use the following criteria for offering targeted fragile X screening or referral for diagnostic testing:

•Individuals seeking reproductive counseling who have a family history of fragile X syndrome (confirmed FMR1 premutation or full mutation), intellectual disability, or autism spectrum disorder of undetermined etiology.

•Individuals of either sex with intellectual disability or developmental delay of undetermined etiology, or autism spectrum disorder.

•Young females with elevated levels of follicle-stimulating hormone, especially with a family history of premature ovarian insufficiency (menopause before age 40), fragile X syndrome, or a relative of either sex with intellectual disability of undetermined etiology.

•Individuals with late-onset intention tremor or ataxia (usually after age 50), especially with a history of infertility, family history of movement disorders, fragile X syndrome, or intellectual disability of undetermined etiology.

●Broad genetic carrier screening panel approach – For individuals without a family history or specific ethnicity considerations, after pretest counseling we now offer an American College of Medical Genetics and Genomics (ACMG) tier 3 panel (inclusive of fragile X) to attain equity in screening across an increasingly varied general population [25]. (See 'Pretest counseling' below.)

ACMG now promotes a tiered approach (tiers 1 to 4) to genetic carrier screening with progressively more conditions included in the higher tiers [25]. The tiers were designed with a goal of equity across a diverse heritage population in the United States. Fragile X is included in tier 3, with ACMG recommendations to offer tier 3 or 4 in either the preconception or prenatal settings. Carrier panels designed for one population (such as persons of Ashkenazi Jewish descent) are highly sensitive and specific but perform less well in the general United States population with increased admixture of cultures, ethnicities, and races. Broader carrier screening panels, such as ACMG tiers 3 and 4 (as opposed to tiers 1 and 2), include a wide range of inherited conditions with better representation of a general population with increasingly diverse heritage.

●Is there a role for universal screening? – Some authorities suggest routinely offering fragile X carrier laboratory screening to all females, given the high test sensitivity (99 percent), the potential impact of the full mutation in offspring, and the relatively high premutation carrier rate (ranging from 1 in 257 for females with a negative family history of intellectual disability, developmental problems, or autism spectrum disorder, to 1 in 86 for females with a positive family history [26]) [11,27]. Concerns about general population reproductive screening for fragile X syndrome include the complexities of pre- and post-test counseling, the challenges in predicting expansion of the premutation, counseling regarding health risks of premutation carriers, and the broad phenotype in the full mutation female fetus. Each of these concerns is undergoing continued review as reproductive genetic carrier screening expands for other conditions.

Universal fragile X carrier laboratory screening is not recommended for males. Their male offspring will not inherit their X chromosome, though all daughters will carry a premutation. Further CGG expansion does not occur in sperm, and some evidence suggests premutations become smaller when passed from a father to his female offspring [18,19].

Pretest counseling — Pretest counseling remains an essential component of all reproductive genetic carrier screening and can be performed virtually, in person, and/or with written materials. Pretest counseling for genetic carrier screening should address:

●Potential variation in the identified condition

●Possible identification of a parental condition with medical implications, and

●Inability to detect all genetic alterations contributing to a specific condition

These three considerations are particularly notable for fragile X carrier screening. An important component is making sure that patients understand that the prognosis of a female fetus with a full mutation remains difficult to predict, given the spectrum of presentation from normal to classic fragile X syndrome and lack of available phenotypic predictors. Secondly, fragile X carrier screening is unique in the association of specific clinical manifestations with premutation carriers (female and male) [28]. Notably, FXPAC is undergoing extensive clinical and biologic investigation to further refine the molecular basis, clinical variability, and contributors to phenotype.

Laboratory testing — The gold standard for fragile X testing is polymerase chain reaction (PCR) followed by Southern blot:

PCR can determine the size of the CGG region. It can accurately identify a normal allele and premutation and assess size differences between large premutations and small full mutations. However, as the region expands to a larger, full mutation, PCR often is unable to discern the margins of the expansion accurately. In such cases, Southern blot is performed to further refine size and, importantly, the methylation status. Southern blot is not performed as the first-line test because it requires substantially more DNA than PCR and is more time consuming to accomplish.

Given the noted limitations with PCR and Southern blot testing, modifications and alternatives to PCR testing continue to emerge. A triplet repeat-primed PCR (TP-PCR) modification provides comparable detection to the two-step PCR and Southern blot testing process. In 2020, a commercial PCR-based interpretation of methylation test (AmplideX Fragile X Dx and Carrier Screen Kit) became available for screening for fragile X premutations in various populations [29-31]. This PCR-based methylation assessment has been reported to be as effective as Southern blot analysis. Intended initially for premutation carrier screening, use as a diagnostic test is being pursued. An alternative approach has also emerged with optical genome mapping (OGM), which may be able to overcome the challenges of multiple, repeated short segments by using labeled, ultra-long DNA molecules. Initial work on FMR1 was reassuring, supporting further development [32].

AGG trinucleotide genotyping may be performed, as it may be useful for counseling females with intermediate and premutation expansions [10]. (See 'FMR1 expansion during gametogenesis' above.)

It is important to note that routine investigations for developmental delay/intellectual disability/autism spectrum disorder, such as chromosome microarray and exome or whole genome sequencing, do not typically detect fragile X syndrome. The diagnosis is usually only made when the clinician requests a specific test that measures the number of repeats in the CGG segment of FMR1. However, genomic sequencing targeting expanded DNA regions is becoming commercially available.

Post-test counseling — Referral to a genetic counselor or other clinicians with expertise in the genetics and clinical findings in fragile X syndrome is appropriate, given the complexity of the disorder.

●Females with a premutation are informed of the risk of having an offspring with a full mutation (table 2). In two cases where the mothers had 59 CGG repeats and one case with 56 CGG repeats, the offspring had a full mutation [9,11,33].

Females with an intermediate number of repeats (45 to 54 CGG repeats) may have a 6.6 percent risk for having an offspring with a premutation, but are not at risk for offspring with a full mutation [11].

AGG determination may further refine the risk of disease in offspring of females with intermediate or premutations as it predicts expansion from a premutation to a full mutation and is one variable in fetal disease prognostication for premutation carriers. As discussed above, no AGG interspersion permits destabilization and increases the likelihood of expansion, whereas two to four AGG interspersions appear to decrease the risk [13]. However, even the maximum number of AGG repeats does not eliminate the risk that a premutation will expand to a full mutation [34].

●Females with a premutation or full mutation are informed of the potential phenotypes of their male and female offspring (table 1) and potential sequelae of premutations and full mutations for future generations. These phenotypes depend on whether the fetus inherits a premutation or full mutation, the sex of the fetus, mosaicism, and the methylation status of the FMR1 gene. Rarely, the number of repeats and the methylation status are discordant, which makes prediction of the phenotype more difficult.  

●Males are informed that they will transmit their X chromosome only to their female offspring. A full mutation is passed on as a premutation. A premutation is usually passed on as a premutation, but contraction may occur: One-third of female offspring who inherit a premutation from their father undergo contraction in the size of the expansion. Female offspring with a premutation are at increased risk for FXPACs and also at risk for CTG repeat expansion to full mutation in their offspring.

●Males and females with a premutation are also informed of their own risk for developing FXPACs. Typical features may include tremor-ataxia syndrome in late adult life (usually after age 50) and, in females, premature ovarian insufficiency (menopause before age 40). In addition, implications for family members should be discussed as they are also at risk of being carriers.

The clinical features of fragile X syndrome and prognosis and management of affected individuals are discussed in detail separately. (See "Fragile X syndrome: Clinical features and diagnosis in children and adolescents" and "Fragile X syndrome: Management in children and adolescents".)

PRENATAL (FETAL) DIAGNOSIS

Candidates for prenatal (fetal) diagnosis — We offer prenatal (fetal) diagnosis for fragile X syndrome to prospective parents in whom the mother or father has >55 CGG repeats and choose this approach after risk counseling with a genetic counselor or other clinician with expertise in the genetics and the clinical findings of fragile X syndrome as well as the clinical implications of premutations.

As discussed above, the degree of risk in offspring depends on several factors, including the sex of the offspring, whether the mother or father is the carrier, the absolute CGG repeat size, mosaicism, and the number and dispersion of AGG repeats. There are potential clinical sequelae to offspring and future generations when either parent is a carrier, although the most severe offspring phenotype only occurs when the mother is the carrier. Offering prenatal (fetal) diagnosis to females with as few as >55 CGG repeats, however, is a conservative approach since few cases (at least three) of expansions to full mutations arising from repeats at this lowest level in females have been reported. (See 'Prenatal (fetal) diagnosis' above.)

Offering prenatal (fetal) diagnosis when the father is a carrier is also a debated approach since his male offspring will not inherit his X chromosome, though his female offspring will be premutation carriers in the absence of contraction. Prenatal diagnosis is rarely performed when the father is a carrier.

Procedure — Molecular diagnostics for fragile X expansion can be applied to fetal DNA, whether obtained from chorionic villus samples (CVS) at 11 to 13 weeks of gestation or amniocytes obtained by amniocentesis at ≥15 weeks of gestation.

A disadvantage of CVS is that assessment of methylation may not be possible at an early gestational age. Methylation of the promoter of FMR1 region occurs typically at 11 weeks of gestation but can be variable. While haplotype testing can track which maternal X chromosome is inherited by the fetus, methylation of the expanded region is a secondary event and its exact timing is controversial. Even in individuals with a full mutation, there can be mosaicism within their tissues for the extent of methylation. Follow-up amniocentesis later in gestation may be considered to determine methylation status to better predict the offspring phenotype if a large premutation or small full mutation is identified.

Determining the exact size of the expansion region by analyzing cell-free DNA in a sample of maternal blood is currently unreliable but may be possible in the future with advances in this methodology.

Interpretation of findings at prenatal diagnosis — Given the complexity of this disorder, the results of prenatal testing and offspring prognosis should be provided to parents in consultation with a genetic counselor or other clinicians with expertise in the genetics and clinical findings in fragile X syndrome (table 1). (See "Fragile X syndrome: Clinical features and diagnosis in children and adolescents".)

The National Fragile X Foundation is an adjunctive resource that provides detailed information on fragile X on its website.

PRECONCEPTION REPRODUCTIVE OPTIONS

●Donor gametes – Prospective parents may avoid conceiving a pregnancy with an affected fetus by:

•Using donor gametes from an unaffected donor

•Avoiding pregnancy

●Preimplantation genetic testing – Prospective parents in whom the female carries a premutation or a full mutation may avoid implantation of a pregnancy with an embryo of either sex with either a full mutation or a premutation by undergoing preimplantation genetic testing (PGT), but PGT for this condition has numerous technical challenges [35]. The polymerase chain reaction (PCR) and Southern blot techniques used for carrier testing and prenatal diagnosis are not always informative in PGT because premutations and full mutations of the FMR1 allele are highly refractory to PCR amplification of DNA from only a single cell, and a single cell does not contain sufficient DNA for Southern blot analysis. In addition, female carriers may have impaired ovarian reserve, which hinders the ability to harvest an adequate number of oocytes for in vitro fertilization and subsequent embryo transfer, all of which are necessary components of PGT [36,37]. However, examination of maternal alleles for markers closely linked to the FMR1 allele can provide helpful information for determining which of the X chromosomes has passed to her male or female offspring. By examining each maternal X chromosome for variations in gene markers (short tandem repeats [STR]) closely linked to FMR1, the STR marker results of the mother become a surrogate for the fragile X site. Such an approach can be used before methylation changes occur, such as in preimplantation embryos and chorionic villus sampling (CVS) analysis of first trimester trophoblast tissues [38]. (See "Preimplantation genetic testing".)

●Noninvasive fetal sex determination – Some females with a premutation may choose noninvasive fetal sexing (eg, cell-free DNA screening, ultrasound of fetal genitalia) to inform their decision about whether to proceed to invasive testing by CVS as in females the effects of a full mutation are so variable (covering the full spectrum from no discernible phenotype through mild cognitive impairment to full fragile X syndrome). If the risk of expansion to a full mutation is low and only a minority of females with a full mutation would be severely affected, some prospective parents may elect not to proceed to invasive testing by CVS/amniocentesis if the fetus is female.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Fragile X syndrome".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Fragile X syndrome (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Genetic basis – Fragile X syndrome is an X-linked dominant disorder associated with a wide spectrum of clinical features, including intellectual disability, neuropsychiatric manifestations, autism spectrum disorder, and seizures. Changes in a region of the X chromosome, known as the fragile X messenger ribonucleoprotein 1 gene (FMR1), lead to the characteristic clinical features. (See 'Introduction' above and 'FMR1 gene and protein' above.)

●Phenotype – Fragile X syndrome is associated with a variety of physical, behavioral, and cognitive abnormalities that vary according to the mutation state (full mutation versus premutation), sex (males with a full mutation are more likely to have intellectual disability than females), degree of FMR1 methylation (hypermethylation worsens outcome), and variation of gene expression across tissues due to mosaicism, as shown in the table (table 1). (See 'Factors affecting phenotype' above.)

●Pathogenesis of the phenotype

•Fragile X syndrome is primarily caused by expansion in the number of cytosine-guanine-guanine (CGG) repeats within the FMR1 gene. Expansion of CGG repeats allows hypermethylation of FMR1, resulting in impaired transcription and reduced production of the fragile X messenger ribonucleoprotein (FMRP), which adversely impacts prenatal and postnatal brain development. (See 'Fragile X syndrome' above.)

•The lower and upper boundaries of repeat length are variably defined, but generally described as (see 'Fragile X syndrome' above):

-Normal – 5 to 44 CGG repeats

-Intermediate expansion – 45 to 54 CGG repeats

-Premutation – 55 to 200 CGG repeats

-Full mutation – >200 CGG repeats

•As repeat size increases above the normal range, stability decreases and further increases in the number of repeats in the FMR1 region become likely. The risk for FMR1 expansion when the gene is passed from parent to child depends on the repeat size (table 2), sex of the parent (expansion only occurs from maternal transmission), and frequency of adenine-guanine-guanine (AGG) trinucleotide interspersion. (See 'FMR1 expansion during gametogenesis' above.)

●Preconception/prenatal screening – Preconception or prenatal screening for fragile X syndrome may be targeted to high risk individuals or offered as part of broad genetic carrier screening panel. (See 'Screening' above.)

•If a targeted approach is used, individuals at increased risk of an abnormality of the FMR1 gene include (see 'Screening' above):

-Individuals of either sex with intellectual disability, developmental delay, or autism spectrum disorder.

-Individuals seeking reproductive counseling who have a family history of fragile X syndrome (confirmed premutation or full mutation of FMR1 gene), undiagnosed intellectual disability, or autism spectrum disorder.

-Young females with elevated levels of follicle-stimulating hormone, especially with a family history of premature ovarian insufficiency, fragile X syndrome, or a relative of either sex with undiagnosed intellectual disability.

-Individuals with a family history of late-onset intention tremor or ataxia, especially with a family history of movement disorders, fragile X, or undiagnosed intellectual disability.

●Counseling patients about test results – After screening, referral to a genetic counselor or another clinician with expertise in the genetics and clinical findings in fragile X syndrome is appropriate.

•Females with a premutation are informed of the risk of expansion to a full mutation in offspring. This risk may be altered by the presence of AGG regions interspersed within the expanded region.

•Males with a premutation or full mutation are informed that their female offspring will have a premutation, although contraction may occur.

•Males and females with a premutation or full mutation are informed of the potential phenotypes of their male and female offspring (table 1) and potential sequelae in future generations.

•Males and females with a premutation are also informed of their own risk of developing FXPAC and referral for appropriate resources provided. In addition, implications for family members should be discussed as they are also at risk of being carriers. (See 'Post-test counseling' above.)

●Prenatal (fetal) diagnosis – Prenatal (fetal) diagnosis for fragile X is offered when the mother has a premutation or full mutation. Fetal testing is performed on cells obtained by chorionic villus sampling or amniocentesis. For males with either a premutation or full mutation, fetal testing is rarely done as all female offspring are expected to be premutation carriers. (See 'Candidates for prenatal (fetal) diagnosis' above and 'Prenatal (fetal) diagnosis' above.)

The results of fetal testing and offspring prognosis should be provided to parents in consultation with a genetic counselor. (See 'Interpretation of findings at prenatal diagnosis' above.)

●Reproductive options – Preimplantation genetic testing is an option to enable transfer only of embryos at lowest risk of fragile X syndrome. (See 'Preconception reproductive options' above.)