"We conclude that women with preeclampsia harbor cellular fetal microchimerism more commonly and at higher concentrations compared with women with uncomplicated pregnancy. The functional capacity and phenotype of these fetal cells are not yet known."http://www.ncbi.nlm.nih.gov/pubmed/24101661
So more of the fetal cells get into our circulation than in normal pregnancy, and we don't know why they're there, or what they're doing. They might be affecting our metabolism, or triggering a stronger immune reaction, or repairing damage from pregnancy, or mucking with our thyroids, or any number of other possibilities.