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Triple X Syndrome

(Trisomy X; XXX)

By Nina N. Powell-Hamilton, MD, Clinical Assistant Professor of Pediatrics ; Medical Geneticist, Sidney Kimmel Medical College at Thomas Jefferson University; Nemours/Alfred I. duPont Hospital for Children

Triple X syndrome 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 triple X syndrome, 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. However, 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.