WASHINGTON, USA (AFP) — Surgery to repair a birth defect known as spina bifida is best done in the womb rather than after the baby is born, the results of an eight-year US trial released Wednesday suggest.

The method was so successful in boosting babies' health and mobility that the trial was halted early, said the findings published 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 buildup 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 Cesarean section to lift the uterus out of the woman's body.

With both mom and fetus under general anesthesia, the uterus is cut open, and an obstetrician positions the fetus's back to the opening so surgeons can stitch it up while the fetus stays inside the uterus.

"The fetus gets an additional shot of muscle relaxant and narcotic," to be sure it is anesthetized, according to Scott Adzick, chief of pediatric surgery at the Children's Hospital of Philadelphia.

"And then the pediatric 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 buildups 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 percent of the prenatal surgery group needed a shunt compared to 82.5 percent in the postnatal group.

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

The Management of Myelomeningocele Study (MOMS) study, a randomized 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 fetuses 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 percent of fetuses who 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 upon being born.

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