Solving a longstanding puzzle, British scientists have finally proved that cancer cells can pass on from the mother to her unborn child.
While there are very rare cases where a mother and child appear to share the same cancer, but in theory the child's immune system should block the cancer.
But, in a new analysis of one such case, scientist have found that the cells, which caused leukaemia in the child, could only have come from the mother.
For a long time, scientists have been trying to find out if it is possible for a mother to "infect" her unborn child with cancer, as theoretically any cancer cells that manage to cross the placenta into the baby's bloodstream should be targeted for destruction by the child's immune system.
However, there are records of 17 cases of a mother and baby appearing to share the same cancer - usually leukaemia or melanoma.
The latest study focused on a Japanese woman and her baby, who both developed leukaemia.
The researchers used an advanced genetic fingerprinting technique to prove that the leukaemia cells found in the baby had originated from the mother and showed that both patients' leukaemic cells carried an identical mutated cancer gene.
But, they also showed that the child had not inherited this gene from its mother, which means that it could not have developed this type of leukaemia in isolation.
They then examined how the cancer cells could have neutralised the baby's immune system and found that the cancer cells lacked some DNA, which played a crucial role in giving them their own specific molecular identity.
Without this telltale molecular sign, the child's immune system was unable to recognise the cells as foreign, and thus was not mobilised to attack them.
"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 foetus and succeeded in implanting because they were invisible to the immune system," the BBC quoted lead researcher Professor Mel Greaves, of the Institute of Cancer Research, as saying.
The study appears in Proceedings of the National Academy of Sciences.