Merck Manual

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Trisomy X

(Triple X Syndrome; XXX)

By

Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision Jul 2020| Content last modified Jul 2020
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Trisomy X is a common sex chromosome abnormality in which girls are born with three X chromosomes (XXX).

Chromosomes are structures within cells that contain DNA and many genes. Genes contain instructions that determine how the body is supposed to function.

The sex chromosomes determine whether a fetus becomes male or female. A pair of X and Y chromosomes (XY) results in a male, and a pair of X and X chromosomes (XX) results in a female.

In trisomy X, 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 is born with the third X chromosome.

Trisomy X rarely causes obvious physical abnormalities. Girls with trisomy X may have slightly lower intelligence, problems with verbal skills, and more school problems than siblings. Sometimes the syndrome causes menstrual irregularities and infertility. However, some women with trisomy X 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.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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