Abstract: The present invention provides, among other things, a simple, reproducible, and cost-effective method for enriching fetal or other low molecular weight nucleic acids in a biological sample. In certain embodiments, methods are provided for enriching fetal nucleic acids (e.g., fetal DNAs), typically comprising steps of adding a polymer such as PEG to a heterogeneous biological sample containing fetal DNA and high molecular weight non-fetal DNA such that the PEG precipitates substantially the high molecular weight non-fetal DNA, and purifying the fetal DNA from supernatant, thereby enriching the fetal DNA.