The human body's most primitive stem cells form in the fetal yolk sac and move to the fetal liver before entering the baby's bone marrow during the third trimester of pregnancy. When a baby is born, the placenta and umbilical cord still contain a rich supply of immature blood stem cells in the very process of migrating. These stem cells are capable of rebuilding all three types of blood cells in the body (red blood cells, immune cells, and platelets). First used in 1989 to help children with leukemia, cord-blood transplants have since proven themselves effective for adults too.