AFAIR sometimes it may happen that familiar stem cells cross maternal-fetal barrier in placenta, persist somehow and start to function regardless, where stem cells are needed - usually in younger sibling coming from the older, in place of original cells, even in the brain - forming part of it as of another person (more or less) - interconnected but not the same..
Wait, shouldn't these cells be rejected like a donor organ?
This is way above my level but a quick skim seems to suggest that not only are the cells not rejected, but that they can actually enhance the recipient's immune system.