In the early weeks of pregnancy, a connection is made between the mother and her growing baby as they exchange cells across the placenta. This little-known phenomenon will affect both of them for decades to come. Microchimerism is when cells from one individual persist in another. The term means a “small chimera”; in Greek mythology, the Chimera is a creature whose body is composed of different animals. Pregnancy is the primary cause of natural microchimerism. Fetal microchimerism—the transfer of fetal cells to the mother—begins in the first month after conception, often before a woman knows she is pregnant. The mother’s immune system removes some of these cells, but those that persist are absorbed into her body and will remain with her for decades, or possibly for life. The study of microchimerism is still in its infancy, but we know that fetal microchimerism benefits the baby and mother alike. Benefits to …