Routine amniotic fluid alphafetoprotein assay: experience with 40,000 pregnancies.

Of 40,000 initial amniotic fluid samples, 0.7% had an alphafetoprotein (AFP) level greater than or equal to +3 standard deviations above the mean. The risk of an open neural tube defect (NTD) or other serious fetal abnormality was 61% when AFP levels measured greater than or equal to +3 and 87% for levels greater than or equal to 5 standard deviations. If the acetylcholinesterase (AChE) was positive, this risk was 84% and 95% for AFP levels of greater than or equal to +3 and greater than or equal to +5 SD, respectively, and 29% and 62% for the same levels if the AChE was negative. If the second amniocentesis AFP was also greater than or equal to +3, the risk of an abnormality was 96%. In this series, there were 93 open neural tube defects, and all were identified. The true false-positive rate was 1 per 13,000 of the cases screened. The recurrence risk for an NTD was 1.8%, and the incidence of NTDs in women having amniocentesis only for maternal age was 1.5 per 1,000.