A woman has transmitted cancer to her unborn daughter during pregnancy in the first proven case of the disease being passed from mother to child while in the womb.

The Japanese woman, who was 28 when she gave birth, has since died from medical complications during her treatment for leukaemia, but her daughter, who is now nearly three, is in remission, scientists said yesterday.

It is believed cancerous white blood cells passed across the placenta to the foetus from the woman and then established themselves in the blood system of the baby, who developed a swelling in her cheek 11 months after her birth.

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Scientists at Britain's Institute of Cancer Research in Sutton, Surrey, were involved in carrying out the analysis of the cancer cells from both mother and child, which used an advanced form of genetic fingerprinting to show that the two cancers were derived from the same source.

It is the first time a cancer has been transmitted in the womb, although there have been about 30 other instances worldwide of mothers sharing the same kind of cancer as their infants, according to Professor Mel Greaves, who led the research team at the cancer institute.

"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 [baby's] immune system," Professor Greaves said. "We are pleased to have resolved this long-standing puzzle. But we stress that such mother-to- offspring transfer of cancer is exceedingly rare and the chances of any pregnant woman with cancer passing it on to her child are remote."