"Guideline for Obstetrical Practice in Japan 2008" was published under the joint edition and supervision of Japan Society of Obstetrics and Gynecology, and Japan Association of Obstetricians and Gynecologists in April 2008. It includes the item about Neural Tube Defects (NTDs) and folic acid supplementation. This guideline is composed of a Clinical Question and several Answers. The Question regarding folic acid is "If you are asked about relationship between NTDs and folic acid, what do you explain?" The answers are as follows. 1. Explanation is the following. (Recommendation Level B) 1) Periconceptional supplementation of 0.4mg folic acid per day is expected to reduce the incidence of NTDs. 2) If women with a previous child with NTD take 4mg folic acid (or one 5 mg-tablet) per day prior to conception under the supervision of a doctor, the reduction for recurrence of affected infants is expected. I hope this guideline will contribute to increasing the folic acid supplementation in women of reproductive potential. This guideline will be revised every 3 years.