Klinefelter syndrome is a genetic disease that causes biological males to be born with an extra copy of the X chromosome in their cells. Males with this condition may not have any symptoms, or may have a variety of symptoms, including smaller-than-normal testicles that produce less testosterone.

Testosterone is the male hormone that stimulates sexual traits like body hair and muscle growth.

A lack of testosterone can cause symptoms, such as:

  • breast growth
  • a smaller penis
  • less facial and body hair than typical

It’s possible that males with this condition may have issues with fertility. Klinefelter syndrome can also delay speech and language development in male children.

The symptoms of Klinefelter syndrome can vary. While some people will have no discernible symptoms and have no idea they’re living with the syndrome, others may deal with a variety of symptoms.

Symptoms in babies

When present, many of the symptoms of Klinefelter syndrome in young children and babies may go overlooked because they aren’t always obvious — especially because child development can be very individualized. However, you may want to talk with your child’s pediatrician if these symptoms are present and concern you:

  • a delayed start in learning to crawl or walk
  • a delayed start in talking
  • a seemingly quieter, or more passive attitude

Symptoms in young males and teens

The symptoms of Klinefelter syndrome are often most obvious in tween and teen males due to the onset of puberty. It’s important to follow your instincts if you, or they, are worried something is amiss. Some of the symptoms of the Klinefelter syndrome in tween and teens are:

  • mild dyslexia
  • issues with paying attention
  • lowered energy levels
  • difficulty socializing
  • delayed puberty, which can include:
    • a lack of facial or body hair
    • a higher pitched voice
    • more fat deposits around the hips and buttocks, creating a rounder lower half of the body
  • enlarged breasts (gynecomastia)
  • taller stature than the rest of the family (including long arms and legs)
  • difficulty increasing or producing muscle tone
  • smaller testicles
  • smaller penis

Symptoms in adults

The main symptom in adults is often fertility difficulties — although there are many treatments available for this symptom.

Adult males who only have an extra X chromosome in some of their cells will have milder symptoms. A smaller number of males have more than one extra X chromosome in their cells. The more X chromosomes they have, the more severe their symptoms may be.

Symptoms of more severe Klinefelter types include:

  • major problems with learning and speech
  • poor coordination
  • unique facial features
  • bone problems

Klinefelter syndrome is one of the most common chromosome conditions in newborns. It affects between 1 in 500 and 1 in 1,000 newborn males. Variants of the disease with three, four, or more extra X chromosomes are less common. These variants affect 1 in 50,000 or fewer newborns.

It’s possible that Klinefelter syndrome affects even more male children and adults than statistics suggest. Sometimes symptoms are so mild that they go unnoticed. Or symptoms can be misdiagnosed as being caused by other similar conditions.

Everyone is born with 23 pairs of chromosomes, or 46 chromosomes in total, inside each of their cells. These include two sex chromosomes, X and Y.

  • People who are born female have two X chromosomes, XX. These chromosomes give them sexual traits like breasts and a uterus.
  • People who are born male have one X and one Y chromosome, XY. These chromosomes give them traits such as a penis and testicles.

People with Klinefelter syndrome are born with an extra X, causing their cells to have XXY chromosomes. This happens randomly during conception. About half the time the extra chromosome starts in the mother’s egg. The other half of the time it comes from the father’s sperm.

Some people with the syndrome have more than one extra X chromosome. For example, their chromosome might look like this: XXXXY. Others have different chromosome abnormalities in each cell. For example, some cells might be XY, while others are XXY. This is called mosaicism.

Klinefelter syndrome is usually random and not caused by anything either parent did.

Females who have pregnancies after age 35 are slightly more likely to give birth to a baby with Klinefelter syndrome.

Many people with Klinefelter syndrome won’t have to worry about complications. But in rare cases, people with this syndrome have an increased risk of:

A small number of males with Klinefelter syndrome receive a diagnosis before birth when their mother has one of these tests:

  • Amniocentesis. During an amniocentesis, a technician removes a small amount of amniotic fluid from the sac surrounding the baby. The fluid is then examined in a lab for chromosome problems.
  • Chorionic villus sampling. Cells from tiny finger-like projections called chorionic villi in the placenta are removed during a chorionic villus sampling. These cells are then tested for chromosome problems.

Because these tests can increase the risk for a miscarriage, they usually aren’t done unless the baby is at risk for a chromosome problem. Often, Klinefelter syndrome isn’t discovered until a child reaches puberty or later.

If your child seems like they’re developing slowly and it feels concerning, talk with your child’s pediatrician. They may refer you to an endocrinologist. This doctor specializes in diagnosing and treating hormonal conditions.

During the exam, your doctor will ask about any symptoms or development issues you or your child has. The main test used to diagnose Klinefelter syndrome is:

  • Chromosome analysis. Also called karyotyping, this blood test checks for abnormal chromosomes, such as an extra X chromosome.

Mild symptoms often don’t need to be treated. Males with more obvious symptoms are often encouraged to start treatment as early as possible, preferably during puberty. Starting treatment early can prevent some of the symptoms.

One of the main treatments is testosterone replacement therapy. Taking testosterone around the time of puberty will trigger the development of characteristics that normally happen at puberty for people who are born biologically male, such as:

  • a deeper voice
  • hair growth on the face and body
  • increased muscle strength
  • penis growth
  • bone strengthening

You can take testosterone as a pill, patch, or cream. Or you can get it by injection every 2 to 3 weeks.

Other treatments for Klinefelter syndrome include:

  • speech and language therapy
  • physical therapy to improve muscle strength
  • occupational therapy to help you function day to day at school, work, and in your social life
  • behavioral therapy to help gain social skills
  • educational assistance
  • counseling to deal with emotional issues like depression and low self-esteem that could stem from the condition
  • surgery (mastectomy) to remove extra breast tissue
  • fertility treatments

Most people with Klinefelter syndrome don’t produce much or any sperm. A lack of sperm can make it harder to biologically have a child, but it’s not impossible.

While having a biological child is not the only way to become a parent, fertility treatments can help some people who are interested in that method.

If you have a low sperm count, a procedure called intracytoplasmic sperm extraction with intracytoplasmic sperm injection (TESE-ICSI) removes sperm directly from your testicle and injects it into an egg to increase the chances of pregnancy.

Many people living with Klinefelter syndrome either don’t know they have it or aren’t adversely affected by it. However, in some cases, living with this syndrome can create stress. Children may feel embarrassed about the lack of changes in their bodies during puberty. Adults may be upset to know their chances of having biological children are low.

A therapist or counselor can help you manage any depression, low self-esteem, or other emotional issues that stem from this condition.

You may also look for a support group, where you can talk with others who have this condition. You can find support groups through your doctor or online.

Children with Klinefelter syndrome often need extra help in school. Contact your local school district to find out about special programs for kids with disabilities. You might be able to get an Individualized Education Program, which helps tailor a learning program to suit your child’s needs.

Many children with Klinefelter syndrome have more trouble interacting socially than their peers. An occupational or behavioral therapist can help them learn social skills.

Most people with Klinefelter syndrome live long, healthy lives. If you or your child requires treatment, the earlier it’s received, the better. This is because treating Klinefelter syndrome during puberty can lessen some of the symptoms.

Like many health conditions, some of the difficulties that arise with a Klinefelter syndrome diagnosis can be mental or emotional. Support from family members, therapists, and loved ones can be just as important as finding the right medical team.