Scientists have suspected for 100 years that a mom with cancer can pass it to her unborn child, but until now, the theory wasn’t possible to confirm genetically. The placental barrier usually protects the baby, and any cells that cross into the baby’s blood stream should be destroyed by its immune system.

The proof came when the daughter of a Japanese mother who was diagnosed with leukemia shortly after giving birth, also developed leukemia eleven months later. Genetic tests showed that cancer genes from the baby had special markers that proved they came from her mother. Other tests showed that the baby’s cells lacked genetic material that would have flagged them as cancerous by her immune system and targeted them for destruction.

Since the placenta usually is a barrier to passing cells from mother to child, mother-to-fetus transmission is probably rare. “Some 30 times reported in the past, mother and infant have appeared to share the same cancer, usually leukemia or melanoma,” lead researcher Dr. Shuki Mizutani of the department of pediatrics at Tokyo Medical and Dental University told HealthDay.

The researchers believe that the baby’s missing genetic material probably allowed the mother’s cancer cells to pass through the placenta. “It appears that in this and, we presume, other cases of mother-to-offspring cancer, the maternal cancer cells did cross the placenta into the developing fetus and succeeded in implanting because they were invisible to the immune system,” lead researcher Mel Greaves of the Institute of Cancer Research, told the BBC.