Earlier, I read this article about fetal microchimerism perhaps helping to protect women from breast cancer. During pregnancy a small number of fetal cells cross into a woman's bloodstream and take up residence in the mother's body. This is known as fetal michrochimerism. These cells persist for years after pregnancy probably for the rest of the mother's life. Somehow, this idea delights me. I find it comforting that I will always carry small bits of all my children. Just a geek, I guess.
Oh, and maternal cells cross into the fetus, so our children carry little bits of us too.