Safe use of valproic acid during pregnancy

I have an epileptic patient who plans pregnancy. It took us years to control her seizures, and valproic acid seems to be the only way to control them. What is the risk to her fetus?

Neural tube defects (NTDs) are the most common of the major anomalies associated with in utero exposure to valproic acid. About 1% to 2% of exposed fetuses suffer adverse effects.