Cell-free DNA molecules in the plasma of pregnant women exhibit nonrandom fragmentation with preferred end sites. We studied if such preferred end sites might bear any re- lationship with fragment lengths of plasma DNA. Short and long plasma DNA molecules were associated with different preferred DNA end sites. Analysis of size-tagged preferred ends could be used for measuring fetal DNA fraction and for facilitating fetal trisomy 21 detection. Fetal preferred end sites were generally located in the nucleosome cores, while the maternal ones were located in the linker regions. This con- ceptual framework provides an explanation of the relative shortness of fetal DNA in maternal plasma and brings us closer to understanding the biological mechanisms that influence plasma DNA fragmentation.