Mothers pass myriad things to their unborn babies via the placenta, and scientists have found that cancer is among them. Researchers studied the case of a mother who died of leukemia soon after she gave birth, and whose child was diagnosed with cancer months later. Though the baby’s cancer type was different, the Guardian reports, both mother and child contained an identical mutated gene that could not have been inherited.
Therefore, researchers say, the cancer must have passed between mother and child physically. The process was confusing to scientists—the child’s immune system should have detected and destroyed the threat—until they found a mutated gene in both cancers that makes them effectively invisible to the body’s defenses. “The maternal cancer cells did cross the placenta into the developing fetus and succeeded in implanting,” a researcher says. “We are pleased to have resolved this longstanding puzzle.”