May also be called: Trisomy 21

Down syndrome (DS) also called Trisomy 21, is a condition in which a person is born with an extra chromosome. Chromosomes contain hundreds, or even thousands, of genes. Genes carry the information that determines your traits (features or characteristics passed on to you from your parents). With Down syndrome, this extra chromosome causes delays in the way a child develops, mentally and physically.

More to Know

Normally a baby inherits genetic information from its parents in the form of 46 chromosomes: 23 from each parent. In most cases of Down syndrome, a child gets an extra number 21 chromosome — for a total of 47 chromosomes. This extra genetic material is responsible for the physical features and developmental delays of Down syndrome.

It is not known why Down syndrome happens, but scientists do know that women age 35 and older have a higher risk of having a child with the condition, and the risk increases with age.

Some characteristic features of people with Down syndrome include a flat facial profile, an upward slant to the eyes, small stature, low muscle tone, and a single deep crease across the palm. Most children with DS reach developmental milestones later than other kids, but the degree to which a person is affected by DS can vary greatly.

Most, but not all, people with DS will experience other significant health problems related to Down syndrome such as congenital heart defects; respiratory, vision, and hearing problems; Alzheimer's disease; childhood leukemia; and thyroid disorders. Fortunately, many of these conditions are treatable.

Keep in Mind

People with Down syndrome have varying degrees of intellectual difficulties, but they are quite capable of developing loving relationships and have many talents to share. Medical advances also allow people with DS to live longer, fuller lives than ever before.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.