Maternal serum alpha-fetoprotein screening for chromosomal abnormalities: a prospective study in women aged 35 and older.

Objective: Our purpose was to determine the detection and false-positive rates for maternal serum alpha-fetoprotein measurement to screen for fetal Down syndrome and other chromosomal abnormalities in women > or = 35 years old.
Study Design: A total of 3896 women had serum maternal serum alpha-fetoprotein levels measured routinely before amniocentesis for the indication of advanced maternal age.
Results: Eighty-five percent (28/33) of fetal Down syndrome pregnancies had second-trimester risks of > or = 1:270 on the basis of a combination of maternal serum alpha-fetoprotein measurement and maternal age. Risks were also > or = 1:270 in 63% of the unaffected pregnancies. Sex chromosome aneuploidies, translocations, and other nonautosomal chromosome abnormalities in this study population were not associated with altered maternal serum alpha-fetoprotein levels; 51.9% (14/27) of these, however, were also assigned risks of > or = 1:270.
Conclusions: Maternal serum alpha-fetoprotein screening is more accurate than age alone for assigning individual Down syndrome risk in pregnant women > or = 35 years old. Counseling for women in this age group should include information regarding the lower sensitivity of maternal serum alpha-fetoprotein screening for detecting fetal Down syndrome and other chromosomal abnormalities (especially sex chromosome aneuploidies) compared with offering amniocentesis to these women.