Abstract Epidemiologic monitoring is a process which uses data generated from the screened population as a quality control measure. Most commonly, laboratories monitor the percentage of women with screen positive test results, i.e., with test results falling at or above a specified multiple of the median (MoM) for open neural tube defect screening or, with a risk at or above a specified risk cut-off for Down's syndrome screening. These percentages are sensitive to inaccurate and imprecise assays, inappropriate reference data, and long term assay drift. In fetal Down's syndrome screening it is necessary to simultaneously monitor two or more assays, which is accomplished by monitoring the median MoM for each assay (which should be 1.0 within statistical limits) calculated using patient MoM values. Monitoring the median MoM can identify inappropriate adjustment factors for variables which affect the MoM such as maternal weight. Epidemiologic monitoring is a useful tool for identifying problems which are not readily apparent using traditional quality control measures.