Typically, humans have 23 pairs of chromosomes. Women with triple X syndrome, as the name implies, have an extra X chromosome. This creates a chromosome constitution of 47,XXX instead of the characteristic 46,XX. This chromosomal change is typically a random event that occurs during cell division; triple X syndrome is not an inherited condition. Triple X syndrome, also known as trisomy X or XXX syndrome, is diagnosed through genetic karyotyping, a test to identify the number of and changes to chromosomes. This entry describes how triple X syndrome is manifested across the lifespan in women who are affected.

Triple X syndrome was first described by cytogeneticist Patricia Jacobs in 1959. Jacobs was asked to look at the chromosome structure of a woman who had stopped menses ...