Cat,
I know this one was some tough going, but I thought it was important to post due to the specific effects on Her2+. Here is a link to a paper that describes pretty well what fetal microchimerism is:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2638004/
Once again, the abstract seems to say that something that is protective against bc in other phenotypes is actually promoting bc in Her2+'s.
Hope this helps,
Hopeful