The finding in 1991 that women who took folic acid supplements prior to conception reduced their risk for having a child with spina bifida or other neural tube defects (NTDs) by more than 50% stimulated an immediate effort to implement this discovery at the population level.1 This effort proved difficult because the neural tube closes by the 28th day after conception, before many women know they are pregnant, and so women may need to begin taking folic acid well before conception.