Fragile X syndrome

Fragile X syndrome is a condition that happens when the body can’t make enough of a protein it needs for the brain to grow and develop. Fragile X can cause problems with learning and behavior. 

Fragile X syndrome is the most common inherited cause of learning problems and intellectual disabilities. In the United States, about 1 in 4,000 boys and 1 in 8,000 girls are born each year with fragile X. Usually, boys with fragile X have more serious learning and behavior problems than girls.

What causes fragile X syndrome?

Fragile X syndrome is inherited. This means it’s passed from parent to child through genes.  A gene is a part of your body’s cells that stores instructions for the way your body grows and works. Genes are stored in structures called chromosomes. Everyone has 23 pairs of chromosomes, or 46 in all. For each pair, you get one chromosome from your mother and one from your father.  

Sometimes the instructions in genes change. This is called a gene change or a mutation. Parents can pass gene changes to their children. Sometimes a gene change can cause a gene to not work correctly. Sometimes it can cause birth defects or other health conditions. A birth defect is a health condition that is present in a baby at birth. 

One set of chromosomes is responsible for your gender. This set is made up of X and Y chromosomes. Girls have two X chromosomes (XX). Boys have an X and a Y chromosome (XY). Fragile X syndrome is caused when part of gene repeats more than usual on the X chromosome. Everyone has at least one X chromosome. If a girl has fragile X, her second X chromosome usually is healthy. But boys with fragile X don’t have a second healthy X chromosome. This is why symptoms of fragile X syndrome usually are more serious in boys than in girls. 

You can have the fragile X gene change and not have clear signs of the condition. When this happens, you’re called a carrier, and the condition is called fragile X premutation. If either you or your partner is a carrier, you can pass fragile X to your baby.

You may be a permutation carrier if you have ovarian insufficiency or fragile X-associated tremor/ataxia syndrome (also called FXTAS). Ovarian insufficiency, sometimes called early menopause, is when your ovaries don’t work correctly or at all at an early age (less than 40 years). FXTAS is a condition that affects your ability to think and move. It usually affects men older than 50, but women can have it.

How can you find out if you’re a fragile X carrier?

Before or during pregnancy, you and your partner may want to have a genetic test called carrier screening to find out if you’re a fragile X carrier. Anyone can have the test. But it may be more important if you: 

  • Have autism or autism-like behaviors. Autism is a group of disorders that affect a person’s speech, social skills and behavior. About 1/3 of males with Fragile X have autism features that affect social skills and behavior. Fragile X is the most common known genetic cause of autism. 
  • Had an unexplained intellectual disability or developmental delay as a child. Unexplained means there’s no known cause (like a head injury) for the disability or delay. A developmental delay is when a child’s skills in movement, learning, thinking, language and behavior develop later than most children’s.
  • Have a family history of fragile X syndrome, ovarian insufficiency, autism or unexplained intellectual disability or developmental delay. Family history means that someone in your family has had one of these conditions.
  • Have a partner who is a fragile X carrier

If you’re thinking about having carrier screening, you may want to see a genetic counselor. This is a person who is trained to help you understand about how genes, birth defects and other medical conditions run in families, and how they can affect your health and your baby’s health. Ask your health care provider if you need help finding a genetic counselor. Or contact the National Society of Genetic Counselors

Can you find out if your baby has fragile X syndrome before birth? 

Yes. If you or your partner is a carrier, you can talk to your health care provider about prenatal tests called amniocentesis and chorionic villus sampling (also called CVS). These tests can help you find out the chances of having a baby with fragile X. 

How do you know if your baby has fragile X after he’s born?

Signs and symptoms of fragile X can be mild or serious. At first, the only sign or symptom you may notice in your baby is a large head. But as he grows, common signs and symptoms can include:

Developmental signs and symptoms 

  • Learning problems, especially for boys
  • Developmental delays in sitting, crawling or walking
  • Problems with speech and language, especially for boys

Social or behavior signs and symptoms

  • Not making eye contact
  • Having trouble paying attention
  • Flapping hands
  • Being hyperactive
  • Being afraid in new situations
  • Being aggressive for boys
  • Being very shy for girls
  • Being bothered by certain sensations, such as bright light, loud noises or the way something feels
  • Not wanting to be touched

Physical signs and symptoms

  • Long face, ears and chin
  • Loose, flexible jointsFlat feet
  • Large testicles after puberty for boys
  • Seizures in about 15 out of 100 boys (15 percent) and 5 out of 100 girls (5 percent)

If your baby has any of these signs or symptoms, or if you have a family history of fragile X or premutation, tell your baby’s health care provider. Your baby can get a blood test for fragile X. 

Boys with fragile X syndrome usually are diagnosed at around age 3. Because girls have less serious symptoms, they’re usually diagnosed a little later, around age 3½.

How is fragile X syndrome treated?

There is no cure for fragile X syndrome. But children with the condition can do well when they get treatment starting in early childhood. They may need help learning to talk, walk and interact with others.

Certain medicines can help some children with behavioral symptoms be better able to learn, including:  

  • Medicines used to treat depression and anxiety and other mental health problems
  • Stimulants like Ritalin®, which often are used to treat attention deficit hyperactivity disorder (also called ADHD)
  • Medicines used to help prevent seizures

More information


Last Reviewed: January, 2014

Fragile X syndrome is a condition that happens when the body can’t make enough of a protein it needs for the brain to grow and develop. Fragile X can cause problems with learning and behavior. 

Fragile X syndrome is the most common inherited cause of learning problems and intellectual disabilities. In the United States, about 1 in 4,000 boys and 1 in 8,000 girls are born each year with fragile X. Usually, boys with fragile X have more serious learning and behavior problems than girls.

What causes fragile X syndrome?

Fragile X syndrome is inherited. This means it’s passed from parent to child through genes.  A gene is a part of your body’s cells that stores instructions for the way your body grows and works. Genes are stored in structures called chromosomes. Everyone has 23 pairs of chromosomes, or 46 in all. For each pair, you get one chromosome from your mother and one from your father.  

Sometimes the instructions in genes change. This is called a gene change or a mutation. Parents can pass gene changes to their children. Sometimes a gene change can cause a gene to not work correctly. Sometimes it can cause birth defects or other health conditions. A birth defect is a health condition that is present in a baby at birth. 

One set of chromosomes is responsible for your gender. This set is made up of X and Y chromosomes. Girls have two X chromosomes (XX). Boys have an X and a Y chromosome (XY). Fragile X syndrome is caused when part of gene repeats more than usual on the X chromosome. Everyone has at least one X chromosome. If a girl has fragile X, her second X chromosome usually is healthy. But boys with fragile X don’t have a second healthy X chromosome. This is why symptoms of fragile X syndrome usually are more serious in boys than in girls. 

You can have the fragile X gene change and not have clear signs of the condition. When this happens, you’re called a carrier, and the condition is called fragile X premutation. If either you or your partner is a carrier, you can pass fragile X to your baby.

You may be a permutation carrier if you have ovarian insufficiency or fragile X-associated tremor/ataxia syndrome (also called FXTAS). Ovarian insufficiency, sometimes called early menopause, is when your ovaries don’t work correctly or at all at an early age (less than 40 years). FXTAS is a condition that affects your ability to think and move. It usually affects men older than 50, but women can have it.

How can you find out if you’re a fragile X carrier?

Before or during pregnancy, you and your partner may want to have a genetic test called carrier screening to find out if you’re a fragile X carrier. Anyone can have the test. But it may be more important if you: 

  • Have autism or autism-like behaviors. Autism is a group of disorders that affect a person’s speech, social skills and behavior. About 1/3 of males with Fragile X have autism features that affect social skills and behavior. Fragile X is the most common known genetic cause of autism. 
  • Had an unexplained intellectual disability or developmental delay as a child. Unexplained means there’s no known cause (like a head injury) for the disability or delay. A developmental delay is when a child’s skills in movement, learning, thinking, language and behavior develop later than most children’s.
  • Have a family history of fragile X syndrome, ovarian insufficiency, autism or unexplained intellectual disability or developmental delay. Family history means that someone in your family has had one of these conditions.
  • Have a partner who is a fragile X carrier

If you’re thinking about having carrier screening, you may want to see a genetic counselor. This is a person who is trained to help you understand about how genes, birth defects and other medical conditions run in families, and how they can affect your health and your baby’s health. Ask your health care provider if you need help finding a genetic counselor. Or contact the National Society of Genetic Counselors

Can you find out if your baby has fragile X syndrome before birth? 

Yes. If you or your partner is a carrier, you can talk to your health care provider about prenatal tests called amniocentesis and chorionic villus sampling (also called CVS). These tests can help you find out the chances of having a baby with fragile X. 

How do you know if your baby has fragile X after he’s born?

Signs and symptoms of fragile X can be mild or serious. At first, the only sign or symptom you may notice in your baby is a large head. But as he grows, common signs and symptoms can include:

Developmental signs and symptoms 

  • Learning problems, especially for boys
  • Developmental delays in sitting, crawling or walking
  • Problems with speech and language, especially for boys

Social or behavior signs and symptoms

  • Not making eye contact
  • Having trouble paying attention
  • Flapping hands
  • Being hyperactive
  • Being afraid in new situations
  • Being aggressive for boys
  • Being very shy for girls
  • Being bothered by certain sensations, such as bright light, loud noises or the way something feels
  • Not wanting to be touched

Physical signs and symptoms

  • Long face, ears and chin
  • Loose, flexible jointsFlat feet
  • Large testicles after puberty for boys
  • Seizures in about 15 out of 100 boys (15 percent) and 5 out of 100 girls (5 percent)

If your baby has any of these signs or symptoms, or if you have a family history of fragile X or premutation, tell your baby’s health care provider. Your baby can get a blood test for fragile X. 

Boys with fragile X syndrome usually are diagnosed at around age 3. Because girls have less serious symptoms, they’re usually diagnosed a little later, around age 3½.

How is fragile X syndrome treated?

There is no cure for fragile X syndrome. But children with the condition can do well when they get treatment starting in early childhood. They may need help learning to talk, walk and interact with others.

Certain medicines can help some children with behavioral symptoms be better able to learn, including:  

  • Medicines used to treat depression and anxiety and other mental health problems
  • Stimulants like Ritalin®, which often are used to treat attention deficit hyperactivity disorder (also called ADHD)
  • Medicines used to help prevent seizures

More information


Last Reviewed: January, 2014