In unaffected pregnancies the level of hAFP increases rapidly with gestation in maternal serum and decreases in amniotic fluid. Fetal open neural wall defects result in a marked elevation in amniotic fluid levels and a smaller but substantial increase on average in maternal serum levels, particularly at 16-18 weeks. Maternal serum hAFP can be used in screening for neural tube and abdominal wall defects. In contrast, the level is reduced by about one-quarter, on average, in pregnancies affected by Down''s syndrome and can be used as part of multi-marker second trimester screening programmes including free hCGb or intact hCG, with or without unconjugated estriol (uE3) and inhibin A. Maternal serum levels are also reduced on average in pregnancies affected by Edwards'' syndrome. This solid phase, two-site fluoroimmunometric assay is based on the direct sandwich technique in which two monoclonal antibodies are directed against two separate antigenic determinants on the hAFP molecule. The analytical sensitivity is typically better than 0.1 U/mL. All Revvity diagnostic products may not be available in certain countries. For information on availability please contact your local representative.