This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (firstname.lastname@example.org) with numerous contributions by others.
3.a This chromosomal disorder is characterized by varying degrees of mental
retardation and an increased risk of physical defects. It occurs in about
one in six hundred births. The risk increases with the age of the mother,
but women of any age can give birth to a baby with Down's. Until recently
it was thought that a woman's age was the only indicator of Down's risk.
Measuring the amount of AFP, or better, the triple screen, and using this
information as well as age will give a more accurate estimate of the risk
of delivering a baby with Down's.