Routine amniotic fluid alpha-fetoprotein measurement in 34,000 pregnancies

Of 34,000 initial amniotic fluid samples, 0.7% had an alpha-fetoprotein level greater than or equal to +3 SD above the mean. The risk of an open neural tube defect or other serious fetal abnormality was 60% when alpha-fetoprotein levels measured greater than or equal to +3 and 86% for levels greater than or equal to +5 SD. In this series there were 72 open neural tube defects, and all were identified. The true false positive rate was 0.9 per 10,000 cases screened.