Try looking up microchimerism. There is some stuff on wikipedia. Just a couple of weeks ago I went to a lecture about placentas given by a guy who works at the Centre for Trophoblast Research at Cambridge University (http://www.trophoblast.cam.ac.uk/
). Placentas are amazing! In a completely non-woo way I hasten to add. Anyway, it turns out that not only do these stem cells (which originate from the placenta) reside in the mother's body, but in subsequent pregnancies some of these foetal cells will make their way into the new foetus - so if you have any older siblings, you have some of their cells in your body. The guy giving the talk also said that people originally thought that these cells were linked in some way to autoimmune disorders, because the cells would turn up in places like the thyroid in people with thyroid disorders. The lecturer said that the current evidence seems to be pointing towards these cells have a role in tissue repair in these conditions.