“To our knowledge, this is the first clinical trial using stem cells in the womb,” says Cecilia Götherström of the Karolinska Institute in Stockholm, Sweden, and coordinator of the Europe-wide trial. “A few cases have been done before, including by us, but there has been no proper trial.”

Brittle bone disease, or osteogenesis imperfecta, is caused by mutations in the gene for making collagen – a tough, flexible material that strengthens bone. Götherström and her colleagues will inject fetuses around 20 weeks old with stromal stem cells containing unmutated copies of the collagen gene.

The stem cells come from the livers of terminated fetuses. Once injected into the recipient, the stem cells divide and become incorporated into bone, where the collagen they produce helps strengthen the developing bones and fix any fractures . “They home to any site of injury,” says Anna David of University College London, one of the other centres involved in the trial.

Starting next January, the researchers aim to treat 15 fetuses and 15 babies with the condition. By comparing the number of fractures in each group, they should be able to determine whether the earlier treatment is more beneficial.

Transplanted stem cells are more likely to be accepted by a fetus than by a baby, because its immune system has not yet developed fully. This also means larger doses can be administered.

All participants will receive booster doses every six months until they are 2 years old. Götherström says she is hopeful of good results since she has already treated two fetuses with the condition. Now aged 2 and 13, the treatment helped them to grow better, without any rejection of the transplanted cells.