James Wilkinson reports on a medical breakthrough

Noah Kipfmiller's leg movement was saved by the operation, and surgeons are confident he has also avoided the mental impairment usually associated with severe forms of the disease.

His mother Mellissa was just 23 weeks pregnant when the procedure was carried out.

Dr Scott Adzick explains the theory behind the operation

Spina bifida affects one in 2,000 babies and can lead to lifelong and severe physical disabilities, including paraplegia.

In many cases of spina bifida the nerves of the spinal cord are exposed, and become damaged because they are not protected by bone and skin.

Noah Kipfmiller: underwent surgery before he was born

This damage is initiated by the mother's amniotic fluid while still in the womb.

Scientists previously thought the damage was caused at a very early stage of pregnancy.

But studies in sheep have shown that the function of the legs can be saved if the nerves are protected by surgery while still in the womb before damage can occur.

In human beings, ultrasound examinations of babies with large spina bifida lesions early in pregnancy show that their legs move normally, whereas later in pregnancy leg movements are lost.

By correcting the lesion in time, leg function can be preserved.

Spina bifida lesion closed after surgery

Dr Scott Adzick and colleagues report in this week's Lancet medical journal that they successfully undertook such a procedure on baby Noah who was found to have spina bifida on an ultrasound scan done at 20 weeks of pregnancy.

The baby's leg movements were still good, despite the extent of the spina bifida lesion, and the operation was done three weeks later.

Noah was born seven weeks after the operation when the mother went into early labour.

The baby's leg movements were good at birth, and he has developed normally during his first six months of life.

Surgery also prevented the formation of fluid around the brain that usually accompanies spina bifida in newborns. This has to be drained by inserting a shunt into the skull, and usually causes brain damage.

Dr Scott Adzick: carried out the surgery

Dr Adzick and his team have extensive experience of operating on foetuses to correct life-threatening conditions such as giant lung masses, and congenital diaphragmatic hernia.

But this was the first time that they had attempted to correct a condition that was not life-threatening.

Dr Adzick said: "We have a great deal of preservation of the function of the legs. The child has a club foot on his right leg, which should respond to exercise therapy, but his left leg appears to function well."

Dr Adzick said it was too early to tell whether bowel and bladder function had been affected, and whether neurological function was unimpaired.

But he said: "We are very excited about it. He seems like a bright little guy.

"The contrast with other children who have not had the operation is quite striking."

Most cases are aborted

In the UK 18,000 people are known to suffer from spina bifida. Approximately 1,000 pregnancies are year are affected, and in nine out of 10 cases the pregnancy is terminated.

Tony Britton, publicity manager of the Association for Spina Bifida and Hydrocephalus, said: "It is too early yet really to claim there is a miracle cure for spina bifida, but it would be nice to think there might be.

"Certainly if the surgical team in Philadelphia can produce further results like this it would look very promising.