When Julie Potter-Tate and Rob Franklin discovered at a routine 20-week scan that their unborn child had a hole in his diaphragm they were faced with a stark choice. Julie could continue with the pregnancy knowing that her child had only a 10% chance of surviving or she could have an abortion.

Pioneering surgery being carried out on babies in the womb at King’s College hospital in London, led by Professor Kypros Nicolaides, offered the couple another option. The experimental surgery could attempt to repair the defect while their son was still developing in the womb.

An abnormal opening in the baby’s diaphragm causes the stomach to move into the chest, often crushing the lungs. The operation itself was dangerous, in that it could prompt a premature labour, causing their child to be stillborn. But Daniel, the baby, survived and was born in July at 34 weeks.

Operations to heal defects on the foetus remain highly contentious, even though it is 25 years since Michael Harrison conducted the first successful operation on an unborn child at the University of California, San Francisco.

The surgery gained international fame several years later when, during an operation in 1999 to repair baby Samuel Armas while he sat in his mother’s womb, the 21-week-old foetus reached out through his mother’s abdomen to grasp the finger of Joseph Bruner, the surgeon carrying out the operation. The photograph of the foetus, clinging on to life, went around the world.

At that time, surgeons had to cut open the womb to operate on the baby. Now, microscopic instruments are used in what is effectively “keyhole surgery”.

In a recent breakthrough at Queen Charlotte’s and Chelsea hospital in London, doctors carried out the first operation on a foetus to create a hole that was missing in the wall of the heart. Surgery on the organ, which was the size of a small grape, allowed blood to flow between two chambers.

In a another procedure pioneered at the hospital, similar to angioplasty in adults, a wire with a “balloon” on the end is fed through the mother’s stomach wall and into the womb. Using this method, the team, led by Dr Helena Gardiner, a foetal cardiologist, have removed blockages in valves in the hearts of several unborn children over the past four years.

At the moment, only around two heart operations are carried out on foetuses per year in Britain but Gardiner believes that up to 60 unborn babies could benefit annually if the procedure became routine. “We have had technical success. Each time we have carried out the operation, we have been able to achieve what we set out to do,” she said. “We would like to carry out more of these operations.

“But this does depend on the opinion of other health professionals. If they do not think the operation is worthwhile, they will not refer the patients.”

At King’s College hospital, one of the greatest successes has been an operation to save identical twins who share a placenta, resulting in one taking an unequal share of the blood, threatening the death of at least one. Surgery for this condition, called twin to twin transfusion syndrome, is now common.

But surgery to heal spina bifida while the unborn child is still in the womb has proved disappointing. Research has shown that the controversial operation does not cure the baby of paralysis: trials are trying to establish whether it does help to prevent the child developing a mental handicap.

Dr Michael Harrison, the international pioneer of surgery on babies in the womb, says we should not be put off: “Of course there have been setbacks in foetal surgery but progress has been fabulous. There are many diseases for which foetal surgery is established and works.”

Nicolaides is also embarking on a trial to show whether diaphragmatic hernia, the operation carried out on Daniel, is worthwhile other than in the most extreme cases where the foetus would not survive.

In the days after Daniel was born he needed further surgery to treat his diaphragm and lungs. At that stage doctors realised how serious his condition had been.

But as he now makes steady progress in a special baby care unit, his parents are convinced that he would not have been born alive had the surgery not been carried out. “I couldn’t have gone ahead with a termination. And because Daniel had only a 10% chance of surviving if we did nothing, I couldn’t leave it to chance,” said Julie.

“The only option for me was to have the surgery. I was very scared, however, not for my own health but the possibility that it could prompt a premature labour and ended the pregnancy there and then.”