According to the Centers for Disease Control and Prevention (CDC), approximately 6,000 babies are born in the United States each year with Down syndrome.1
Down syndrome is the most frequent chromosomal cause of mild to moderate intellectual disability, and it occurs in all ethnic and economic groups.2
Researchers know some, but not all, of the risk factors for Down syndrome. For example, parents who have a child with Down syndrome or another chromosomal disorder, or who have a chromosomal disorder themselves, are more likely to have a child with Down syndrome.3
In the United States, demographic factors also affect the risk for a child to be born with Down syndrome. These factors include geographic region, maternal education, marital status, and Hispanic ethnicity.4,5
Because the likelihood that an egg will contain an extra copy of chromosome 21 increases significantly as a woman ages, older women are much more likely than younger women to give birth to an infant with Down syndrome. Although women older than 35 years of age make up a small portion of all births6 in the United States each year, about nearly one-half of babies with Down syndrome are born to women in this age group.4
This likelihood increases as age increases. The CDC provides a breakdown of the risk for Down syndrome by mother's age at https://www.cdc.gov/ncbddd/birthdefects/downsyndrome/data.html.