Maternal serum alpha-fetoprotein screening: low and high values for detection of genetic abnormalities.

In initiating a maternal serum alpha-fetoprotein screening program, we pursued not only elevated values for the detection of neural tube defects but also low values to detect trisomic fetuses. We detected neural tube defects (0.2%) as expected but were surprised by the efficacy with which low serum alpha-fetoprotein values identified aneuploid fetuses. In 1421 pregnant women, 132 (9.3%) showed maternal serum alpha-fetoprotein values less than 0.4 multiple of the median. After repeat sampling, 57 women still had low values. These 57 women and six others who were too anxious for repeat sampling underwent level I ultrasound examination with the following results: gestational age overestimated by 2 weeks (n = 8), fetal death (n = 1), and no explanation (n = 54). Of the 54, 49 underwent amniocentesis with detection of three aneuploid fetuses: trisomy 18 (n = 2) and trisomy 21 (n = 1); maternal ages were 27, 29, and 31 years, respectively. Autosomal aneuploidy did not occur in other women screened. We conclude that low maternal serum alpha-fetoprotein values could efficiently detect aneuploid fetuses, perhaps with greater sensitivity than previously predicted.