The most common neural tube defects are spina bifida(an incomplete closure of the spinal cord and spinal column), anencephaly (severe underdevelopment of the brain), and encephalocele (when brain tissue protrudes out to the skin from an abnormal opening in the skull). All of these defects occur during the first 28 days of pregnancy - usually before a woman even knows she's pregnant.
That's why it's so important for all women of childbearing age to get enough folic acid - not just those who are planning to become pregnant. Only 50% of pregnancies are planned, so any woman who could become pregnant should make sure she's getting enough folic acid. (and women planning on becoming egg donors and surrogates)
Doctors and scientists still aren't completely sure why folic acid has such a profound effect on the prevention of neural tube defects, but they do know that this vitamin is crucial in the development of DNA. As a result, folic acid plays a large role in cell growth and development, as well as tissue formation.