A new study reported in Proceedings of the National Academy of Sciences shows that it is possible for cancer cells to pass from a mother to her unborn baby in the womb.

The study found proof of this from the fact that in one case, involving a Japanese woman and her baby, the leukaemia causing cells in the baby could only have come from its mother.

The previous thinking had been that any cancer cells that might go into the child's bloodstream would be blocked by the child's immune system.