New research in the United States shows that babies with spina bifida recover better if they are operated on in the womb, rather than after they are born.
Spina bifida occurs when the spine does not develop properly, causing possible paralysis below the waist as well as neurological problems.
Foetal surgery has been carried out since 1997 but, until now, there has been no clear evidence that the benefits outweigh the risks, not just for the babies but for their mothers too.
Babies who have the operation in the womb are more likely to be able to walk without help, and less likely to need a drainage tube in their brains.
But the research did highlight some potential problems – 80 percent were born around a month early, and a third of the mothers suffered complications from the birth.