fetal microchimerism

Having a child changes a woman. It turns her into a mother … 
but also into a kind of chimera.

This transformation doesn't result in a part-lion, part-goat
fire-breathing monster (chimera) of Greek mythology.
But it does result in another type of chimera —
one defined by the presence of cells in the body
originating from two organisms. A mother may live
the rest of her life with cells in her body that are not her
own, but her baby's.

During pregnancy, some of the fetus's cells leave the
womb, traveling through the placenta and into the
mother's bloodstream, where they end up in various
parts of her body.