Down syndrome is a genetic condition caused by abnormal cell division in the egg, sperm, or fertilized egg. This results in an extra or irregular chromosome in some or all of the body's cells, causing varying levels of intellectual disability and physical problems.

Down syndrome is also called trisomy 21, for the specific chromosome that has the abnormality. A person with Down syndrome has three copies of chromosome 21. Normally, a person has two copies.

Down syndrome usually can be detected during pregnancy or soon after birth. Chromosome tests and how a baby looks can help make a diagnosis.