BoingBoing has a fascinating summary of how fetal cells invade their mother's host body and get up to all kinds of shenanigans: some good, some bad.
Research describing the fetal cell invasion has peppered the popular press in recent years, and it's a fascinating story. The cells might protect mothers from cancer, heal their hearts, and help in wound healing. Or, the cells might actually be lowering mothers' immune defenses and causing cancer. Or both!