ALMOST 90% of women could be carrying their children's DNA in their blood during pregnancy, and up to 50% for decades afterwards, in a condition called microchimerism.

New Scientist magazine reported last year that doctors in Boston found a 52-year-old woman's twin sons were not able to donate a kidney to her.

Genetic tests suggested the woman's sons could not be hers. But closer analysis found she had a condition which has only been reported about 30 times. She was a tetragametic chimera.

When her mother was pregnant, non-identical twins fused together in the womb to make one person.

Scientists say many other chimeras probably go through life unaware of their genetic make-up. Some do display physical signs, however, such as having different coloured eyes.

During pregnancy cells can slip between the foetus and the mother.