The discovery of cell-free RNA in plasma and serum samples provides possibilities for noninvasive prenatal diagnosis. Quantitative alterations of cell-free placental-derived mRNA in the maternal circulation are associated with many pregnancy-related disorders, such as preeclampsia and preterm labor. Obtaining circulating cell-free RNA is the first and often the most critical step for analyzing the placental-derived mRNA in maternal blood. We have compared different protocols for the extraction of cell-free RNA from plasma samples, and we have found the protocol using TRIzol LS reagent (Invitrogen) as lysis buffer combined with RNeasy Mini kit (QIAGEN) to be the optimal method for extracting high-quality cell-free RNA in the highest quantities possible. This method is also amenable to the simultaneous extraction of cell-free RNA from many different samples, by use of the QIAGEN Vacuum Manifold QIAvac 24 Plus.