One of the most common chromosomal abnormalities that occurs when a baby has an extra copy of chromosome 21. Women over the age of 35 are at higher risk of having a baby with Down syndrome. About 1 in 1,250 babies are born with Down syndrome to women who are 25 years old; approximately 1 in 400 babies born to women who are 35 have Down syndrome; 1 in 100 are at risk for the syndrome when the mother is 40. Symptoms range from mild to severe, but people with the syndrome usually develop slower mentally and physically than those without the condition. Also called Trisomy 21.