Probability of Having a Baby with Down Syndrome

Down syndrome (trisomy 21) occurs in approximately 1 in 700 births in the United States, making it the most common chromosomal condition. About 6,000 babies are born with Down syndrome each year. The condition is caused by an extra copy of chromosome 21.

The risk increases dramatically with maternal age: approximately 1 in 1,250 at age 25, 1 in 400 at age 35, 1 in 100 at age 40, and 1 in 30 at age 45. However, because younger women have more babies overall, about 80% of children with Down syndrome are born to women under 35.

Prenatal screening tests (first-trimester combined screening, cell-free DNA testing) can detect Down syndrome with high accuracy during pregnancy. Cell-free DNA testing has a detection rate exceeding 99% with a very low false-positive rate. Life expectancy for people with Down syndrome has improved dramatically, from about 25 years in 1983 to over 60 years today, thanks to improved cardiac surgery, medical care, and community integration. Most people with Down syndrome have mild to moderate intellectual disability and can live semi-independently with appropriate support.