Dr Joe Bruner, from Vanderbilt University Medical Center in Nashville, Tennessee, is the leading light in foetal surgery, a highly controversial technique which is highlighted in a BBC1 documentary on Wednesday.

Only a handful of surgeons worldwide are prepared to carry out foetal surgery - and only a few hundred babies have undergone the procedure.

Living Proof: Born Twice follows the progress of US mothers who have undergone the procedure, including Kathy Gunnells, whose unborn child has been diagnosed with spina bifida.

Spinal damage

In a normal baby, the spinal cord is protected by the bones of the spine, but in spina bifida, part of this bony protection is missing, leaving the nerves exposed and vulnerable.

Dr Joe Bruner: "A major advance"

The condition, if left untreated, can cause permanent damage, leading to paralysis.

In the UK, the established practice is to operate on the baby almost immediately after birth and place a protective layer of tissue around the spine.

This reduces the damage, although it frequently cannot prevent some permanent disability.

But Dr Joe Bruner believes the earlier the procedure can be carried out, the lower the resulting levels of disability.

However, there is a strong risk that the operation could trigger premature delivery, which could leave the baby at a high risk of dying.

Dr Bruner says: "This is a real dilemma that parents face. If it's reasonable to repair the lesion before delivery than it probably makes even more sense to repair it as early as possible.

"So parents understand that the earlier the surgery is done, the better likelihood of a good outcome. So they're trying to balance the potential benefit of doing the repair early against the risk of premature delivery."

His results are encouraging, if not conclusive.

Surgeon Tony Hockley, from Birmingham Children's Hospital, says that more proof that the benefits outweighed the risks would be needed before the procedure could be routinely introduced in the UK.

The foetal spine is revealed by cutting through the wall of the womb

"I think colleagues in the UK would wish to see more evidence that foetal surgical repair carried real benefits."

In the case of Kathy Gunnells, the surgery was carried out in the 23rd week of pregnancy, giving her baby only a one in five chance of survival should birth be triggered by the trauma of the operation.

She says: "With this operation I'm just hoping that it'll give him more of a normal life. Definitely not make him perfect but as close to being a normal child as he can be."

The operation itself is complex - involving making a cut and pulling out the womb from the abdomen, without separating it from the body.

The fluid surrounding the foetus is removed and stored, and a second cut through the womb made to access the spine.

A neurosurgeon then tries to repair the damage and encase the tiny spine in a protective sheath of tissue, before the fluid is returned, the womb stitched up and placed back into the patient.

All the time the surgeons guard against triggering the contractions which will lead to premature delivery.

Previous patients testify to the success of the operation.

Alex and Joyce Garcia's son, Nicholas, had the operation to correct a milder form of spina bifida, and at 18 months, he is now walking.

Mr Garcia says: "If we had to go for that decision again, and again, we would go for that surgery, because we know that it has proved beneficial to our son."