Background: ADAM12 has been shown to be an efficient maternal serum marker for Down syndrome (DS) in the first trimester; but recent studies, using a second generation assay, have not confirmed these findings. We examined the efficiency of a second generation assay for ADAM12. Materials and Methods: ADAM12 concentrations were determined in 28 first trimester DS and 503 control pregnancies using a novel Research DelfiaR ADAM12 kit. Log10MoM distributions of ADAM12 and correlations with other markers were established. Population performance of screening was estimated by Monte Carlo simulation. Results: ADAM12 was significantly reduced in the first trimester in DS pregnancies with a log10MoM of -0.1621 (equivalent to 0.68 MoM) (p < 0.001). The reduction decreased with advancing gestational age. ADAM12 used with PAPP-A+hCGβ+NT (CUB screening) increased the detection rate (DR) from 86% to 89% for a false positive rate (FPR) of 5%. When used for a fixed DR of 90%, the addition of ADAM12 resulted in a 25% reduction of the FPR. Conclusion: ADAM12 is a moderately effective DS marker. It is not a cost-effective addition to CUB screening, but may be used to reduce the FPR in selected high-risk cases.