8. Even if you do not think you ever had a twin, there are many other ways you might be invaded by another human’s cells. It’s possible, for instance, that you started off as two foetuses in the womb, but the twins merged during early development. Since it occurs at such an early age of development, the cells can become incorporated into the tissue and seem to develop normally, yet they are carrying another person’s genetic blueprint. “You look like one person, but you have the cells of another person in you – effectively, you have always been two people,” says Kramer. In one extreme case, a woman was surprised to be told that she was not the biological mother of her two children (See “Brother from another mother”, left). Alternatively, cells from an older sibling might stay around the mother’s body, only to find their way into your body after you are conceived.