A delicate form of surgery performed while a baby is still in the womb could vastly change the outcomes for children born with spina bifida, reveals a long-awaited study.

The study found that spina bifida surgery performed in the womb greatly improves the chances that the child will one day be able to walk independently.

But the study also found that the infants who underwent this prenatal surgery were much more likely to be born premature than the infants who had the surgery after birth, which put them at increased risk for breathing difficulties.

Spina bifida is the most common birth defect of the central nervous system, affecting about 1,700 babies each year in North America.

It's caused by the incomplete closing of the embryonic neural tube, which is the precursor to what becomes the spinal cord and the central nervous system.

Those with the most severe form of spina bifida, called myelomeningocele, frequently suffer lifelong disabilities, including paralysis, bladder and bowel problems, hydrocephalus (excessive fluid pressure in the brain), and cognitive impairments.

Although there is no single known cause of spina bifida, ensuring that an expectant mother has adequate stores of folate before conception and during the first weeks of pregnancy greatly reduces the risk.

The study looked at babies with spina bifida who had surgery to close their spinal column while in the womb, compared to those who had the surgery immediately after birth.

It found that the prenatal surgery babies were able to walk better two and a half years after surgery, compared to children who received the surgery shortly after birth. They also scored better on tests of motor function.

Dr. N. Scott Adzick, the surgeon-in-chief at the Children's Hospital of Philadelphia and the lead author of the study says the surgery is not meant to be a cure for spina bifida.

"I don't want anyone to get the impression we are curing spina bifida before birth. On average, we are making the children significantly better," he told CTV News.

Intrauterine surgery for spina bifida is a new form of surgery and no hospitals in Canada perform it, as they've waited for the results of this study, which appears today in the New England Journal of Medicine.

The study recruited women whose fetuses had been diagnosed with spina bifida and randomly assigned half the women to receive prenatal surgery, and the other half to receive postnatal surgery.

Those getting the postnatal surgery were delivered by planned cesarean section at 37 weeks, and given the surgery within 24 hours after birth.

The prenatal surgery was done between 19 and 26 weeks' gestation. The mothers in this group stayed near the hospital for ongoing monitoring, then underwent delivery by planned cesarean section at 37 weeks.

In both groups, surgeons used the same technique to cover the myelomeningocele with multiple layers of the fetus' own tissue.

Many of the babies in the prenatal surgery group arrived prematurely. In all, 46 arrived before 34 weeks gestation in the prenatal surgery group, compared to just five in the postnatal surgery group

Two babies died before birth in each surgery group; no mothers died.

Almost 160 of the babies were followed for at least one year after surgery. They were then evaluated at one year of age and again at age 30 months by specialists who didn't know which kind of surgery each child received.

-- At one year, 40 per cent of the kids in the prenatal surgery group had received a shunt (a surgically implanted tube that drains fluid from the brain), compared to 83 per cent of those in the postnatal group.

-- At age 30 months, children in the prenatal surgery group had significantly better scores in measurements of motor function. The study found a twofold increase in the proportion of children able to walk without crutches or other assistive devices: 42 per cent in the prenatal group compared to 21 per cent in the postnatal group.

The researchers note that fetal surgery carries risks. The majority of  the infants in the prenatal surgery group were born premature, at an average of 34.1 weeks of pregnancy.

Those early births resulted in 21 per cent of the infants in the prenatal surgery developing "respiratory distress syndrome. Among the postnatal surgery group, only 6.3 per cent had respiratory distress syndrome.

The long-term effects of the increased incidence of premature delivery among the prenatal surgery group are still being monitored.

The prenatal surgery also raised the risk of scarring in the mother's uterus.

"Additionally, the surgical site in the uterus used for fetal surgery requires that the mother will have to undergo a cesarean section for any subsequent births," said study co-author Mark P. Johnson, M.D., obstetrics director at the Center for Fetal Diagnosis and Treatment (CFDT).

For Adzick, who has been working to advance fetal surgery since performing preclinical studies in the early 1980s.

"It's very gratifying to take this idea forward over 30 years, starting with a concept and now offering hope—to families, mothers and the children themselves," he said.