Genetics Northern California

Trisomy X (Triple X syndrome)

What is trisomy X ?

How common is trisomy X?

What causes trisomy X?

How is trisomy X diagnosed?

Can trisomy X be diagnosed during pregnancy?

How is trisomy X treated?

Where can I get more information about trisomy X?

What is trisomy X?

Trisomy X (or triple X syndrome) is a genetic condition that only happens in females. Girls with this condition look like other girls except they may grow slightly taller than expected. Most girls and women with this condition never know they have trisomy X unless special testing is done.

Medical concerns in trisomy X

Most girls with trisomy X are healthy. However, some medical concerns are more likely with this condition:

  • Kidney problems (10% or 1 in 10 girls)

  • Heart defects (10% or 1 in 10 girls)

  • Seizures (15% or 1 in 6 girls)
  • Autoimmune diseases (3% or 1 in 33 girls) 

Learning and behavior in trisomy X

There is a wide range of learning abilities in girls with trisomy X. Intelligence is usually in the normal range; but IQ scores tend to be a little lower than others in the family. Girls with trisomy X may start walking a little later than average. They are also more likely to have speech and language delays. Behavior concerns can include shyness, attention problems, delayed maturity, autism, and difficulty with social skills. Girls with this trisomy X are also more likely to report low self-esteem and poor self-confidence. However, it is not possible to predict which girls with trisomy X will have learning or behavior concerns.

How common is trisomy X?

It is estimated that one in every 1000 newborn girls has trisomy X. In California, that means one girl is born with trisomy X almost every day.

What causes trisomy X?

Trisomy X is a genetic condition, but it is not inherited from either parent. Normally, females have two X chromosomes, and males have one X and one Y chromosome. Girls and women with trisomy X have three X chromosomes instead of two.

Genes are the instructions or blueprints that make each human being unique. Our genes are packaged into chromosomes. Most people have 46 chromosomes. Chromosomes come in pairs, and we inherit one of each pair from our mother and the other from our father. One pair is the called the sex chromosome pair, the X and Y chromosomes. These chromosomes include the genetic instructions for a person’s biologic sex. Sometimes, instead of having two sex chromosomes, a baby is born with one extra sex chromosome. This happens completely by chance, usually when an egg or sperm forms. When there is a difference in the number of sex chromosomes, it is called a sex chromosome condition.

How is trisomy X diagnosed?
This condition can be diagnosed by a chromosome study, usually done on a blood sample.

Can trisomy X be diagnosed during pregnancy?

Trisomy X can be accurately diagnosed during pregnancy by chorionic villus sampling (CVS) or amniocentesis. A prenatal screening test called non-invasive prenatal testing (NIPT) or cell-free DNA testing can also check for trisomy X. However, NIPT often has false positive results for trisomy X, so more testing is needed to be sure.

How is trisomy X treated?
Girls with trisomy X may benefit from early intervention programs, like speech therapy or physical therapy. With early intervention and family support, many girls with trisomy X do just as well as girls without this condition.

Where can I get more information about trisomy X?

  1. Genetic Home Reference: Triple X syndrome - General information about triple X syndrome provided by the National Library of Health. Also includes links to related sites.

  2. AXYS: http://www.genetic.org/ - A support organization for individuals with X and Y chromosome variations

         - Brochure on trisomy X (from AXYS): My Guide to Trisomy X

  3. Unique: https://www.rarechromo.org/- A support organization for individuals with chromosome conditions.

         - Brochures on trisomy X (from Unique)

            For girls: Things that make me special

            For parents: Triple X syndrome also called Trisomy X
            A Guide for Parents: Information for parents who are considering whether to talk about an extra X chromosome with their daughter and family members.


Last reviewed: October 2, 2019

Reviewed by: Regional Genetics Department