Doctors have used ground-breaking surgery on an unborn baby to successfully correct a birth defect while he was still inside the womb.

His mother, Maisie Savage, from London, was operated on by doctors in the US while she was 26 weeks pregnant.

Baby Theo, who is now five months old and thriving, had gastroschisis, where the belly button does not develop properly, causing the intestines to grow externally.

Doctors partially exposed his mother’s womb and then gently pushed his organs back inside his abdomen using keyhole surgery.

He was the third baby in the world to benefit from the first-of-its-kind clinical trial.

Maisie, 29, and Theo’s father, Josh French, 36, both teachers, say they are incredibly relieved by the success of the pioneering surgery.

“He’s a little miracle, isn’t he?” Maisie told BBC News.

But they both know things could have been very different.

The serious birth defect affects one in 3,000 UK babies each year. In the best-case scenarios, babies can require weeks in intensive care and nutritional feeding through a tube or IV and corrective surgery.

For the most complex cases, like Theo’s, outcomes are far more serious with up to six months in neonatal intensive care, multiple surgeries, IV nutritional feeding for up to two years, and a possible intestinal transplant.

Even with the highest level of medical care, one in 10 babies will die.

Theo’s parents initially had no idea anything was wrong with their unborn child.

“Going towards a 20-week scan, we were purely just excited to have an image of our baby,” said Josh. “We didn’t know he was going to be a ‘he’ at the time.”

Josh said discovering Theo had gastroschisis had been a shock and “incredibly scary”.

At 24 weeks gestation, specialist doctors at London’s Great Ormond Street Hospital (GOSH) had been preparing expectant parents Maisie and Josh for what the first six months of Theo’s life were going to look like.

However, after discovering he had the more serious condition of complex gastroschisis, doctors referred him to Texas Children’s Hospital, in Houston.

There, a team of foetal and paediatric surgeons, led by Dr Michael Belfort, had been leading a first-of-its-kind clinical trial to repair complex gastroschisis before birth.

Belfort, the clinical lead on the trial, had previously pioneered a similar surgical procedure in the womb on babies with spina bifida.

Maisie was told she would have to travel to Texas straight away and remain there for the rest of her pregnancy. “It was a huge decision,” she said.

As part of the trial she first had to travel to Leuven Hospital, in Belgium, where specialists needed to give Maisie a Botox injection through her womb into Theo’s abdominal wall, to relax the muscles to make the repair easier.

The family then flew to Texas where the operation was performed in November last year, 26 weeks into the pregnancy.

Josh said he had had an agonising wait while doctors had operated on Maisie. “The length of that day, it felt like a week,” he said. “The waiting and just not knowing exactly what was going on at each and every stage.”

Maisie’s womb was partially exposed to position the uterus and baby, the amniotic fluid was drained and replaced with carbon dioxide (CO2) gas to expand the operating space. Using keyhole surgery, Theo’s organs were then gently pushed back inside his abdomen, which was stitched closed.

Maisie was left with a scar, twice the size of a C-section incision. But unlike other women who are recovering from a C-section, she still had a baby to grow inside her for another 14 weeks.

She said Theo would often kick inside her, hitting her scar and causing her pain. “It was a hard recovery,” Maise said. “But, it was worth it.”

The operation was a complete success and Theo was carried to term and born in February by vaginal delivery in Texas.

Belfort, obstetrician and gynaecologist-in-chief at Texas Children’s Hospital, said he and his team are very happy with the results so far.

“The mother, the baby, the surgery, the time of the surgery, the fact that we had zero complications during the foetal surgery and after the foetal surgery, just remarkable.”

If the trial is a success, Paolo de Coppi, professor of paediatric surgery at GOSH and head of stem cells and regenerative medicine at University College London, would perform the procedure in the UK.

“We hope that we will be able to offer this to patients in a larger scale very soon,” he said.

Belfort is excited about the partnership and said Texas Children’s and GOSH working together will be a formidable team.

And Belfort is already looking ahead to what he can do next.

“The inside of the uterus is now a new surgical space for innovation,” he said. “And spina bifida and gastroschisis are just two, but I believe that we could actually do certain cardiac surgeries in utero, and I think we can also do some lung surgeries in utero.”

Back home in London, Maisie said Theo is doing amazingly.

“Everything they did just completely reversed what could have been a really horrible start to his life and he’s just the best. He’s a completely normal baby essentially.”

“We both feel incredibly fortunate,” Josh said. “The impact that they’ve had on us as a family is incredible. They made us feel so welcome from the team at Great Ormond Street to Dr Belfort and the team.

“Whilst we were over there, the level of care was amazing, and they made a very traumatic time of our lives so much easier, and we’ve got this little man – we couldn’t be happier.”

If you have been affected by any of the issues raised in this story, information and support can be found at the BBC’s Action Line.

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