Women with an extra X chromosome in their cells may show no symptoms of this condition.

Triple X syndrome — also called XXX syndrome, trisomy X, or 47 ,XXX aneuploidy — is a genetic disorder in which a woman carries an extra X chromosome in each of her cells.

Normally, women have two X chromosomes per cell, and men have an X and a Y.

But in triple X syndrome, as its name suggests, three X chromosomes are present.

Sometimes only some of the body's cells contain the extra X chromosome, a form of triple X syndrome called mosaicism.

Although it's a genetic disorder, triple X syndrome (which affects only women) is not usually inherited.

Nonetheless, there's some evidence that the incidence of triple X increases with maternal age (i.e., women who are older when they give birth).

The extra X chromosome usually results from random error when the parents' egg or sperm cell is forming.

In mosaicism, the error happens during cell division early in the fetus's development.

How Common Is Triple X Syndrome?

While triple X syndrome is rare, it might not be as rare as the numbers would suggest.

In the most frequently cited statistic, triple X occurs in about one out of every 1,000 female births, meaning that five to 10 females are born with triple X syndrome each day in the United States.

But since many girls and women show no or very mild symptoms of triple X, some researchers estimate that only 10 percent of the cases are ever diagnosed and that the actual numbers are higher.

Signs and Symptoms of Triple X Syndrome

While the symptoms and physical characteristics associated with triple X syndrome vary widely among girls and women — with some displaying no symptoms or unusual physical characteristics — tall stature is the most common trait.

Other physical, developmental, and behavioral characteristics associated with triple X syndrome include:

Skin folds that descend and cover the inner corners of the eyes

Wide spacing between the eyes

Decreased muscle tone (hypotonia)

Curved fifth ("pinky") fingers (clinodactyly)

Small head circumference

Lower average birth weight

Learning disabilities, such as delayed speech and language development, and difficulty reading

Delayed motor skills, such as sitting and walking

Problems processing sound

Behavioral and emotional problems, including attention deficit hyperactivity disorder (ADHD), anxiety, and depression

Premature ovarian failure or ovary malformations (though infertility is rare)

Menstrual irregularities

Constipation or abdominal pains

Flat feet

Mild concavity of the breastbone

In the mosaic form of triple X syndrome, fewer cells have the extra X chromosome, so symptoms and characteristics, when they are present, are more mild .

Seizures or kidney abnormalities (such as having a single kidney, or a malformed one), are possible in triple X but rare — they show up in about 10 percent of cases.

Most girls and women with triple X syndrome are usually of normal intelligence, but IQ scores, especially verbal scores, tend to be 10 to 15 points below that of siblings or those in control groups, according to the National Organization for Rare Disorders (NORD), especially if learning disabilities are not addressed early.

Heart abnormalities have been noted in some isolated cases, according to NORD.

How Is Triple X Syndrome Diagnosed?

Triple X syndrome can be diagnosed prenatally, through CVS (chorionic villus sampling, in which tissue samples are taken from the placenta) or through amniocentesis.

The condition can also be diagnosed through a post-birth blood test if developmental delays, poor muscle tone, or other physical characteristics associated with triple X prompt a doctor to suspect it.

Treatment and Medication Options for Triple X Syndrome

While the extra X chromosome can of course never be removed, early intervention, such as speech or physical therapy, can help, as well as counseling as girls enter middle school and adolescence.

Early screening for kidney and heart abnormalities, which although rare are possible in triple X, is also recommended.

Women who experience late periods, menstrual abnormalities, or who have difficulty conceiving should be evaluated for primary ovarian failure.