Triple X Syndrome

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Trisomy X, also called triple X syndrome or 47,XXX, is characterized by the presence of an additional X chromosome in each cell. Girls and women with this condition have three X chromosomes instead of the usual two. The signs and symptoms of trisomy X can vary widely. Some people with trisomy X do not appear to have any features of the condition, while others have multiple signs and symptoms. The most common feature of trisomy X is tall stature.

People normally have 46 chromosomes in each cell. Two of the 46 chromosomes, known as X and Y, are called sex chromosomes because they help determine whether a person will develop male or female sex characteristics. Females typically have two X chromosomes (46,XX), and males typically have one X chromosome and one Y chromosome (46,XY).

Trisomy X occurs in about 1 in 1,000 newborn girls. Trisomy X is the most common disorder in girls and women that involves changes in the number of sex chromosomes. Five to 10 babies with trisomy X are born in the United States each day. However, as the signs and the symptoms of this condition are often mild, it is estimated that only 10 percent of affected individuals receive a diagnosis.

Most cases of trisomy X are not inherited. The addition of another X chromosome usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in an egg or sperm cell with an abnormal number of chromosomes. In the case of trisomy X, an egg or sperm cell gains an extra copy of the X chromosome. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of the body's cells and have trisomy X.

Published Date: August 05, 2026
Published By: National Institutes of Health