People with Down syndrome are born with an extra chromosome, which changes the way their brain and body develop.
Down syndrome is a genetic disorder. Most babies are born with 23 pairs of chromosomes within each cell for a total of 46. A chromosome is a structure that contains genes, which are made up of your DNA. Genes determine how you form and develop growth in the womb and after birth. The majority of babies with Down syndrome are born with an extra copy of chromosome 21, with three copies of the chromosome instead of the usual two.

As a result people born with Down syndrome face some physical and mental challenges throughout life. They typically have distinctive body and facial features that set them apart. They are more likely to develop slowly, and are more at risk for some medical conditions.

Down syndrome is the most common chromosome-related condition in the U.S. Roughly 6,000 babies are born with the condition in the U.S. every year (about 1 in every 700 babies). More than 400,000 people are living with the condition in the U.S. today.

Researchers are unsure why Down syndrome occurs in certain people. Anyone of any race or societal level can be affected.

What is known is that the risk of giving birth to a child with Down syndrome increases as women grow older. Women aged 35 and older are more likely to have their pregnancy affected by the syndrome. But because younger women have higher fertility rates, most babies with Down syndrome are born to women under 35.

Each human cell typically contains 23 pairs of chromosomes. Down syndrome occurs because of changes in the way cells in chromosome 21 divide. Every person with Down syndrome has an extra amount of this chromosome in some or all of their cells.

In the most common type of Down syndrome, trisomy 21, the condition occurs randomly and isn’t inherited. However, when translocation or mosaicism is the cause of Down syndrome, hereditary (passed down among family members) should be looked at as a cause.

Common learning and behavioral symptoms of Down syndrome include:

- Delays in speech and language development.
- Attention problems.
- Sleep difficulties.
- Stubbornness and tantrums.
- Delays in cognition.
- Delayed toilet training.
Not all people with Down syndrome have all these symptoms. Symptoms and their severity are different from person to person.