Background: mRNA of placental origin (i.e., human placental lactogen and β-human chorionic gonadotropin) has been demonstrated to be easily detectable in maternal plasma. We tested whether detection of chromosome 21-encoded mRNA of placental origin is possible in maternal plasma obtained during the first trimester. Methods: Plasma samples were obtained from pregnant women between weeks 9-13 of pregnancy. RNA was isolated from 800 or 1600 μL of plasma by silica-based affinity isolation and, after on-column DNase treatment, was subjected to two-step, one-tube reverse transcription-PCR with gene specific primers. Results: Three chromosome 21-encoded genes located within the Down syndrome critical region with overexpression in trisomy 21 placentas were screened for expression in early placental tissue to select their potential use for RNA based plasma screening. One of the chromosome 21-encoded genes (LOC90625) showed strong expression in first trimester placenta similar to CSH1 (human placental lactogen) and was selected for plasma analysis. The RNA isolation assay was validated with CSH1 mRNA, which could be detected in the plasma of all women tested in weeks 9-13 of pregnancy. RNA from the chromosome 21-encoded, placentally expressed gene, LOC90625, was present in maternal first-trimester plasma and could be detected in 60% of maternal plasma samples when 800 μL of plasma was used and in 100% of samples when 1600 μL of plasma was used. Conclusion: The detection of chromosome 21-encoded mRNA of placental origin in maternal plasma during the first trimester may allow development of plasma-RNA-based strategies for prenatal prediction of Down syndrome. LOC90625 is a candidate gene for this purpose.