Fetuses give their parents a unique gift: cells that travel through the placenta and become part of the parent’s body. My cells could be in my mom’s lungs, brain, or heart, and research suggests that my mom also probably gave me some cells, even if it’s only around one in a million. I find it comforting to think that my mom has a family tree inside of her, with cells from me, my older sister, and possibly even my mom’s grandmother.