December 1, 2009 9:19 p.m. EST

Washington, D.C. (AHN) - The number of American babies born with Down syndrome increased 31 percent from 1979 to 2003, according to the Center for Disease Control and Prevention (CDC). The percentage translates to nine to 11 babies with the condition per 100,000 live births.

The CDC attributed the increase to more mothers older than 35 giving birth, wherein the chance of bearing a child with the condition is higher.

Down syndrome occurs when a child has an extra chromosome, number 21 of the 23 that determine genetic characteristics. Babies of older women commonly have this genetic defect.

An estimated 400,000 Americans suffer from Down syndrome. They face retardation, delayed language learning and slow motor development.