Spina bifida is a birth defect in a baby's neural tube. The best preventative method is to take a folic acid supplement in the weeks before and after baby's conceived. It's important to do this if you're trying to conceive because baby's neural tube closes around days 49 to 56 — and often, that's about the time women realize they're pregnant.
You should be taking a minimum of 400 micrograms of folic acid — most prenatal vitamins have about 800 micrograms. Some women who are at higher risk for having a baby with spina bifida, like those with diabetes or who've previously had a baby with a neural tube defect, will be prescribed a higher dose of folic acid (usually about four milligrams — that's 10 times as much). Certain medications, like antiseizure meds, could put your baby at risk for neural tube defects, so be sure to run anything you're taking past your doctor.