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 (karyotype) tests and how a baby looks can help make a diagnosis.

Babies usually have distinctive facial characteristics, such as upward-sloping eyes and a flattened nose. People with Down syndrome have an increased risk of being born with or developing health problems. For example, some babies with Down syndrome are born with heart, intestinal, ear, or respiratory defects. These health conditions often lead to other problems, such as respiratory infections, sleep apnea, or hearing problems. Other health issues, such as vision trouble or problems with thyroid function, can also develop.

Children with Down syndrome grow and develop more slowly than other children. But most are able to attend school, play sports, socialize, and enjoy active lifestyles. Unless their disabilities are severe, adults with Down syndrome can care for most of their own needs. Many people with Down syndrome live into their 50s and some into their 60s or older.