A pregnant woman’s blood stream contains not only her own cells, but a small number of her child’s, as well, and some of them remain in her internal organs long after the baby is born. Understanding the origin and identity of these cells is vital to understanding their potential effects on a mother’s long-term health. For example, fetal cells have been found at tumor sites in mothers, but it is unknown whether the cells are helping to destroy the tumor or to speed its growth.
