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 look and 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.
