What causes Down syndrome?

Down syndrome is caused by problem with a baby’s chromosomes that happens long before the baby is born. Chromosomes are the part of your cells that contain your genetic material, or DNA. Normally, a baby has 46 chromosomes. But most people with Down syndrome have 47 chromosomes. In rare cases, other chromosome problems cause Down syndrome. Having extra or abnormal chromosomes changes the way the brain and body develop.

Experts don't really know what causes the chromosome problem. But some things are known to increase the chance that a baby will have Down syndrome. These things are called risk factors. They include having a:

* Mother who is over 35. The older the mother is, the higher her baby’s risk of Down syndrome. (But most babies with Down syndrome are born to mothers younger than 35, because younger women as a group have more babies than older women do.)
* Father who is over 40.
* Brother or sister with Down syndrome.