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.