A notable degree of research attention is being focused on the use of fetal cells enriched from the blood of pregnant women as a non-invasive means of prenatal diagnosis. By using magnetic activated cells sorting (MACS) and fluorescence in situ hybridization (FISH), we have examined the efficacy of enriched fetal cells in determining fetal sex. An unexpected finding of this investigation was that the sensitivity of this analysis was influenced by the anticoagulant used to treat the maternal blood samples. As such, samples treated with heparin showed significantly lower detection rates than samples chelated with EDTA.