Down syndrome (trisomy 21) is the most common chromosomal abnormality in live births. It occurs in approximately 1 in 700 births. With increasing maternal age, this prevalence increases to about 1 in 400 (at age 35) and 1 in 100 (at age 40). The risk of having a baby with Down syndrome increases with maternal age, however, age cannot serve as the sole screening factor; 70% of babies with Down syndrome are born to women under 35, reflecting the increased number of pregnancies in that age group.
More information on Down syndrome is presented in other sections of this course.