Triple X syndrome is a rare disorder in which female infants are born with three X chromosomes.

The extra X chromosome is usually inherited from the mother. The older the mother is, the greater the chance her fetus could have the syndrome. About 1 in every 1,000 girls are born with the third X chromosome.

Triple X syndrome rarely causes obvious physical abnormalities. Girls with triple X syndrome may have slightly lower intelligence, problems with verbal skills, and more school problems than siblings. Sometimes the syndrome causes menstrual irregularities and infertility, although some women with triple X syndrome have given birth to physically normal children who have normal chromosomes.

Extremely rare cases of infants with four or even five X chromosomes have been identified. The more X chromosomes the girl has, the greater the chance of intellectual disability and physical abnormalities.

Last full review/revision April 2013 by Nina N. Powell-Hamilton, MD