Tetrasomy X is a chromosome disorder that only affects females and is caused by having four copies of the X chromosome instead of two. Females with tetrasomy X have a total of 48 chromosomes in their cells, so this condition is sometimes written as 48, XXXX. The signs and symptoms of tetrasomy X vary, but can include mild to moderate speech and learning difficulties; developmental delay; distinctive facial features; dental abnormalities; hypotonia and joint laxity; radioulnar synostosis; heart defects; hip dysplasia; and problems with ovarian function. An increased risk of childhood infections has also been reported. Tetrasomy X is caused by a random error that occurs during the development of an egg cell and is not caused by anything a mother does during her pregnancy.

Tetrasomy X is usually caused by a random error in the formation of an egg cell (before pregnancy). In some cases, it may be due to inheriting three X chromosomes from the mother and one X chromosome from the father. In other cases, it may be due to inheriting all four X chromosomes from the mother. During the normal formation of egg cells, each egg cell contains one X chromosome to pass on to offspring. However, errors in cell division can cause an egg cell to have three or four X chromosomes, instead of one. If an egg cell with the extra X chromosomes is fertilized by a sperm cell with one X chromosome, the resulting embryo will have these extra chromosomes. Rarely, tetrasomy X may be caused by an error in cell division that occurs after an egg is fertilized, or by the presence of extra X chromosomes in some of the mother's cells.

The main factor known to increase a person's risk to have a baby with an abnormal number of chromosomes is maternal age. Women are born with all the eggs they will ever have. As a woman ages, so do her eggs. Errors may develop in the genetic material of eggs over time. Because of this, as a woman's age increases, the risk to have a baby with a chromosome abnormality increases.

People with specific questions about recurrence risks for themselves or family members should speak with a genetics professional.

If a person is known to have a very increased risk to have a child with a specific genetic abnormality, preimplantation genetic diagnosis (PGD) may be available. To have PGD, a couple must first undergo in vitro fertilization (IVF). Each embryo is tested for the condition, and only the embryos that are not affected are then transferred into the mother.

People interested in having preimplantation genetic diagnosis should speak with their health care provider or a genetics professional.