Down syndrome also known as Trisomy 21, is the most common and readily identifiable chromosomal condition associated with intellectual disability. It is caused by a chromosomal abnormality: for some unexplained reason, an accident in cell development results in 47 instead of the usual 46 chromosomes. This extra chromosome changes the orderly development of the body and brain. The condition is genetic and risk factors increase with the age of a woman who becomes pregnant. There is no prevention for Down Syndrome, though diagnostic tests are available to give parents an indication of the condition prior to birth.