Umbilical cord blood (often called cord blood) is the blood left in the umbilical cord and placenta after the baby is born and the cord is cut. Cord blood usually is discarded along with the umbilical cord and placenta. Cord blood, however, is a great source for stem cells which are used to treat genetic blood and immune diseases. Some people are treated with chemotherapy which destroys their own stem cells. Stem cell transplants are done so the the new stem cells create copies of themselves in the bone marrow to rebuild a new and healthy immune system. umbilical cord blood is now the most common source of stem cells for children requiring a stem cell transplant.