This is amazing :)
Scientists Discover Children's Cells Living in Mothers' Brains
The cells pass through the placenta from fetus to mother, some have been identified in women's brains in autopsy, as part of study about Alzheimers. It found that if you have lots of these "microchimeric cells" your chance of Alzheimers is lessened, as quite possibly your chance of developing cancer. If you don't have so many, you have a higher chance of MS however.
Our own cells also pass from mother to child, through the placenta and also later while BFing, and have been found in several places in their bodies. Twins share cells, and while in the womb, foetuses DCs can even get cells from older siblings (which have resided in the mother since pregnancy, then crossed back through the placenta).
Amazing stuff 
Link to the study