Maternal alpha-fetoprotein screening: two years' experience in a low-risk district.

Over a two-year period, 3479 pregnant women in the Kings' Lynn Health District were screened for neural tube defects by estimation of maternal serum alpha-fetoprotein. Most pregnancies were scanned by sonar for fetal maturity. Eight women had fetuses with open neural tube defects; four with anencephaly were associated with very high alpha-fetoprotein values. Of the four with open neural tube defects without anencephaly, only one was detected by screening and confirmed after amniocentesis. One other had a raised serum alpha-fetoprotein but a normal amniotic fluid value. The other two affected fetuses were missed. This disappointing outcome was attributed to the poor predictive value of alpha-fetoprotein in detecting open neural tube defects (anencephaly apart) rather than to errors in its estimation or in assessment of fetal maturity by sonar scan. We question the validity of screening, particularly in areas of intermediate or low incidence.