Thanks for the link!

A total of 154 girls were studied of which 21 (13.6%) tested positive for male microchimerism. There was a tendency that girls were more likely to test positive for male microchimerism if their mothers previously had received transfusion, had given birth to a son or had had a spontaneous abortion. Furthermore, the oldest girls were more likely to test positive for male microchimerism. However, less than half of microchimerism positivity was attributable to these factors. In conclusion, (this is the important part) data suggest that male microchimerism in young girls may originate from an older brother either full born or from a discontinued pregnancy or from transfusion during pregnancy.

Weird...nothing to do with past sexual partners.