Blood plasma of pregnant women contains fetal and (generally>90%) maternal circulatory extracellular DNA. Most of said fetal DNA contains .Itoreq.500 base pairs, said maternal DNA having a greater size. Separation of circulatory extracellular DNA of .Itoreq.500 base pairs results in separation of fetal from maternal DNA. A fraction of a blood plasma or serum sample of a pregnant woman containing, due to size separation (e.g. by chromatography, density gradient centrifugation or nanotechnological methods), extracellular DNA substantially comprising .Itoreq.500 base pairs is useful for non-invasive detection of fetal genetic traits (including the fetal RhD gene in pregnancies at risk for HDN; fetal Y chromosome-specific sequences in pregnancies at risk for X chromosome-linked disorders; chromosomal aberrations; hereditary Mendelian genetic disorders and corresponding genetic markers; and traits decisive for paternity determination) by e.g. PCR, ligand chain reaction or probe hybridization techniques, or nucleic acid arrays.