The human body is made of millions of cells. Inside the nucleus of each cell is genetic information, found in chromosomes, that determines all of a person's inherited features. Normally, each cell contains 23 chromosomes inherited from each parent, totaling 46 chromosomes per cell. The cell then divides so that all cells contain identical genetic information. A person with Down syndrome has an additional chromosome 21, so that there are 3 chromosomes instead of 2 on chromosome 21.

In addition to altered physical features, Down syndrome is often associated with cardiovascular problems and mental retardation.
There is no known cause for Down syndrome, but research shows that women who give birth over the age of 40 are at an increased risk for having a baby with this genetic disorder.