Surgery to repair a birth defect known as spina bifida is best done in the womb rather than after the baby is born, according to the results of an eight-year US trial.

The method was so successful in boosting babies’ health and mobility that the trial was stopped early, said the findings in the New England Journal of Medicine.

Spina bifida is a disorder of the central nervous system that occurs when the spinal cord is partially exposed, protruding on the baby’s back.

Children may be paralysed or may need braces in order to walk, and they may also experience loss of bladder and bowel function.

Many of the babies who have a severe form, known as myelomeningocele, also have a brain stem defect that causes a build-up of spinal fluid in the brain and requires a permanent shunt to drain it.

Typically doctors wait until after the child is born to do surgery. But prenatal surgery can be done at up to 26 weeks’ gestation, with doctors performing a sort of early Caesarean section to lift the uterus out of the woman’s body.

With both mother and foetus under general anaesthesia, the uterus is cut open, and an obstetrician positions the foetus’s back to the opening so surgeons can stitch it up while the foetus stays inside the uterus.

“The foetus gets an additional shot of muscle relaxant and narcotic” to be sure it is anaesthetised, according to Scott Adzick, chief of paediatric surgery at the Children’s Hospital of Philadelphia.

“And then the paediatric neurosurgeon does the same type of layered repair before birth that is done after birth to protect the exposed spinal cord.”

By operating before the child is born, doctors saw fewer build-ups of brain fluid, better motor skills and greater likelihood that the children would eventually be able to walk without braces.

“The damage to the spinal cord and nerves is progressive during pregnancy, so there’s a rationale for performing the repair by the 26th week of gestation, rather than after birth,” said study co-author Leslie Sutton of the Children’s Hospital of Philadelphia.

Children were evaluated at one year of age and again at age two- and-a-half. At 12 months, just 39.7 per cent of the prenatal surgery group needed a shunt compared to 82.5 per cent in the postnatal group.

At 30 months of age, children who had the surgery in the womb performed better in mental deve-lopment and motor skills, with 41.9 per cent able to walk without crutches or braces compared to 20.9 per cent in the postnatal group.

The Management of Myelomeningocele study, a randomised clinical trial, was meant to enroll 200 pregnant mothers but was stopped about two months ago at 183.

“This is the first time a randomised clinical trial has clearly demonstrated that surgery before birth can improve the outcome for patients,” said the study.

However, doctors noted that the procedure carried heavy risks, including premature birth, and did not work for all patients.

Two foetuses died in the prenatal surgery group – one in the womb and one after being born very prematurely at 25 weeks.

Two babies in the postnatal repair group also died, and those deaths were attributable to their spinal malformations, doctors said.

Eighty per cent of foetuses that underwent prenatal surgery were born premature – at an average of 34 weeks compared to 37 weeks in the postnatal group – and 20 per cent showed breathing problems after being born.

Mothers who underwent the procedure would be forced to have any future deliveries by C-section to avoid uterine rupture.

“Even though the children who underwent the surgery while in the uterus did much better overall, these risks both to the foetus and the mother cannot be ignored,” said Diana Farmer, chief surgeon of Benioff Children’s Hospital at the University of California San Francisco.

The study also noted that the initial location of the spinal malformation had an important impact on the children’s ability to walk, regardless of when they had the surgery.

National Institutes of Health funding will allow for the children to be monitored between the ages of six and nine to see if there are lasting effects.

Doctors said the highly specialised procedure has been performed at UCSF and Philadelphia children’s hospitals, and may soon be offered at hospitals in Michigan, Texas and Ohio.

Spina bifida affects about 1,500 babies born each year in the United States. Folic acid supplements of 400 micrograms daily are recommended for all women of child-bearing age to protect against it.