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Alpha-fetoprotein was measured in 2209 amniotic fluid samples under improved assay conditions. All cases of open neural tube defects had elevated values greater than 5 SD above the mean. The false positive rate based on the 3 SD cutoff is estimated at less than 9.15%. Alpha-fetoprotein assays are recommended for all patients undergoing second trimester amniocentesis, optimally between 14 and 16 weeks' gestation.
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