Mother as Chimera

"... fetal stem cells are tenacious things. They stay in the body of the mother, and have even been shown to slip into that sacred space where we believe that all of human individuality resides; the brain. It's possible that part of every mother's brain literally is that of her child. And if you're a child, it's likely that part of you is your mother. A mother's cells cross the placenta during pregnancy and squat in the liver, bloodstream, thymus gland, and that old sentimental favorite, the heart."

http://io9.com/5911357/theres-a-good-chance-youre-a-human-chimera