Scientists yesterday strengthened the prospects of treating genetic disorders in foetuses as early as 10 weeks old after isolating and treating stem cells from foetal blood.

Conditions linked to abnormalities in nerves, bone, cartilage and muscles, which at present are irreversible, might be prevented long before birth by gene therapy in the womb, according to the researchers at Imperial College, London.

They have successfully inserted a marker gene into foetal cells and proved it responds to manipulation, although they cautioned that treatment in humans was still several years down the line.

Screening methods for Down's syndrome and other chromosomal disorders can already spot abnormalities in foetuses within about three months of conception. But big improvements in ultrasound equipment and needles to collect blood from pregnant women would be needed as well as further advances in knowledge about the sort of genetic transfers necessary to tackle different diseases.

The researchers, who reported the latest developments in the British Journal of Obstetrics and Gynaecology, suggested that cells could eventually be removed from foetuses at eight weeks, worked on in the laboratory, and retransfused back into the foetuses within two weeks.

The cells circulate in the blood of foetuses only between about eight and 14 weeks after conception, suggesting a biological function of spreading the building blocks of the body as fast as possible.

Pioneering treatment begun in Paris and followed at Great Ormond Street hospital and the Institute of Child Health, London, has seen recycling of children's stem cells, taken from their hip bones and modified before transfusion back into their bodies.

Nick Fisk, one of the researchers, said the latest success was only "a very small step".