It's true that folic acid's important puma86. It belongs to one of the vitamin B groups - B9? I forget now, but I think the medical profession has determined that we perhaps don't get enough of this from our intake of food alone, and that's why it's been promoted as a beneficial supplement before, and during pregnancy.
Having said that, I still think that a healthy diet with whole grains, fruit and green leafy veg will, in many cases, provide this vitamin and nourish your growing baby. I'm not saying don't take the folic acid as prescribed, or if you feel this is sensible, but no one knows the exact causes of spina bifida, except that the mother's nutrition can play a part in. This is why folic acid and other vitamins are suggested, to supplement the vitamins that may be missing from a diet which isn't rich in the necessary components.