Maternal serum alpha-fetoprotein, age, and Down syndrome risk.

Before the discovery that an association existed between low maternal serum alpha-fetoprotein levels and Down syndrome, the ability to detect Down syndrome pregnancies was limited to women aged greater than or equal to 35, whose individual risks were sufficiently high (greater than or equal to 1:270 in the second trimester) to justify offering amniocentesis. If such women were to opt for that procedure, about 20% of all cases of Down syndrome could be detected. It is now possible to identify an additional 20% of all Down syndrome pregnancies in women under age 35, with the use of a screening process that combines a woman's maternal serum alpha-fetoprotein level and her age. We present a method whereby a woman's individual odds for Down syndrome can be calculated by combining the maternal serum alpha-fetoprotein measurement with her age, on the basis of published age-related Down syndrome risk data and on maternal serum alpha-fetoprotein distributions for unaffected and Down syndrome pregnancies. These individual odds calculations provide the basis both for establishing maternal serum alpha-fetoprotein/Down syndrome screening cutoffs and for counseling.