1. The critical period for supplementing folic acid. During the fetal period, the brain develops earliest and fastest; early pregnancy (3-6 weeks) is the critical period for the growth and development of the fetal central nervous system. At the end of the fourth week of pregnancy, the embryo forms a primitive brain vesicle. Although the fetus is only about 3 centimeters long and weighs more than 2 grams at the 8th week