For years, surgeons have been trying to find ways of operating on babies in the womb, reasoning that medical abnormalities might be more easily fixed while a fetus is still developing. But with tremendous risks to babies and mothers, and a mixed record of success, fetal surgery is mostly used when babies are likely to die otherwise. Now, for the first time, a rigorous clinical trial shows that fetal surgery can help babies with a condition that is not usually life-threatening. Babies with a form of spina bifida, a debilitating spinal abnormality, were more likely to walk and experience fewer neurological problems if operated on before being born rather than afterward. The $22.5 million study, long awaited by experts and published online Wednesday in The New England Journal of Medicine, is likely to galvanize interest in trying to address problems before birth, including operating on serious heart defects and bladder blockages, and potentially using fetal bone marrow or stem cell transplants for sickle cell anemia and immune disorders.