Prenatal diagnosis of neural tube defects through amniotic fluid measurements.

On the basis of our experience with 1314 prospective diagnoses for neural tube defects, we continue to rely on amniotic fluid AFD as the primary and most reliable indicator. The problem of fetal blood contamination has to be tackled in a quantitative manner, and an attempt made to estimate the likely contribution from fetal serum based on measurements of the proportion of fetal erythrocytes in the mixture. We also recommend estimation of the macrophage proportion at 18 hours in the cultured amniotic fluid cells, and a detailed examination of the morphology of cells that adhere, at this time, to the culture surface. An outline of the procedure that we currently employ in handling amniocentesis samples is shown in Figure 3.