"This is a very promising and quite an exciting result," said Diana Farmer of the University of California at San Francisco, one of three centers that conducted the study.
Based on the findings, published in a paper released online by the New England Journal of Medicine, the researchers told reporters Tuesday that expectant parents who find out their babies have spina bifida should consider the operation.
Other experts hailed the findings, saying it could encourage developing procedures to treat fetuses for other conditions.
"The possibility of surgical repair of fetal anomalies in utero has long tantalized obstetricians and pediatric surgeons," wrote Joe Leigh Simpson of Florida International University and Michael F. Greene of Massachusetts General Hospital in an editorial, calling the study a "major step in the right direction."
Spina bifida is the most common birth defect of the central nervous system. About 1,500 babies are born each year with the most severe form of the condition, known as myelomeningocele. It occurs when part of the spinal column does not close around the spinal cord. Babies born with the condition often suffer a host of severe, lifelong disabilities, including paralysis that renders them dependent on crutches or wheelchairs, along with bladder and bowel problems and excessive fluid buildup in the brain.
When babies with spina bifida are born, surgeons have been inserting the spinal cord back into the spinal column and then sealing the column. In the study - the first to evaluate a fetal operation for a non-life-threatening condition - surgeons performed the two-hour procedure while the baby was in the womb, about 19 to 26 weeks into the pregnancies.
Researchers had planned to test the approach on 200 babies. But the study was stopped in December because the results appeared to be so promising. An average of 21/2 years after the surgery, about 42 percent of those who got the surgery in the womb could walk, compared with about 21 percent of those who got the surgery after they were born.
The surgery did have some risks. The mothers who underwent the procedure were more likely to experience ruptured membranes and deliver their babies prematurely. Babies born prematurely were more likely to experience breathing problems. All women who had the surgery would have to give birth in the future by Caesarean section.
"On balance, infants and moms who underwent fetal surgery did better than those who had surgery after birth," said Alan E. Guttmacher, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which funded the study.
The Children's Hospital in Philadelphia and Vanderbilt University in Nashville also helped conduct the study.