A pregnant woman knows she is shaping her child's future from the moment of conception. But she might not realise that the baby is already talking back.

A pregnant woman knows she is shaping her child's future from the moment of conception. But she might not realise that the baby is already talking back. Mother and child are engaged in a silent chemical conversation throughout pregnancy, with bits of genetic material and cells passing not only from mother to child but also from child to mother. Scientists increasingly think these silent signals from the foetus may influence a mother's risk of cancer, rheumatoid arthritis and other diseases, even decades after she has given birth.

We've known for more than a century that cells from a pregnant woman can make their way through the placenta to an unborn child. Identical twins also can exchange these microchimeric cells through their shared placenta. But it was a surprise when researchers at Stanford University, found a few cells with Y sex chromosomes in a pregnant woman's blood in 1979; those cells had to have come from her son, since women have only X chromosomes.

It turns out that all pregnant women carry some foetal cells and DNA, with up to 6 percent of the free-floating DNA in the mother's blood plasma coming from the foetus. After the baby is born, those numbers plummet but some cells remain. In 1996, Diana Bianchi, a geneticist at Tufts Medical Center, found male foetal cells in a mother's blood 27 years after she had given birth.

Evidence is building that those foetal cells aren't just lounging around in Mom; in fact, they might be active participants in a mother's health. But as research in this new field accumulates, so too do the perplexing contradictions about these rare alien elements.