Down syndrome is a genetic defect 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 cognitive disability (mental retardation) 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. Fetal ultrasounds, chromosomal studies, and a baby's appearance can all be used to 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. However, 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.