Prenatal diagnosis of neural tube defects. I. Problems and pitfalls: analysis of 2495 cases using the alpha-fetoprotein assay.

The alpha-fetoprotein (AFP) assay is now an established tool for the prenatal diagnosis of neural tube defects (NTDs). About 90% of these defects are diagnosable prenatally early in the second trimester, the other 10% consisting of closed lesions that are not amenable to this approach. From the analysis of our 2495 consecutive cases, 49 NTDs were diagnosed with AFP levels 3 SD above the mean. One closed NTD sample, as expected, did not have an elevated AFP level. Various other fetal disorders or conditions were also associated with elevated AFP levels. AFP assays on amniotic fluids from 1858 patients without a family history of NTD and studied primarily for fetal karyotyping yielded a frequency of 1 NTD in 310 cases. Any amniotic fluid study in the second trimester of pregnancy should include an assay for AFP.