Vitamin B9, also known as folic acid, is a B vitamin necessary for cell replication and growth. Folic acid helps form building blocks of DNA, which holds the body's genetic information, and building blocks of RNA, needed for protein synthesis. Folic acid is most important, then, for rapidly growing tissues, such as those of a fetus, and rapidly regenerating cells, like red blood cells and immune cells. Folic acid deficiency results in an anaemia that responds quickly to folic acid supplements.

The need for folic acid increases considerably during pregnancy. Deficiencies of folic acid during pregnancy are associated with low birth weight and an increased incidence of neural tube defects in infants. Most doctors, many other health-care professionals, and the March of Dimes recommend that all women of childbearing age supplement with 400 mcg per day of folic acid. Such supplementation may protect against the formation of neural tube defects during the time between conception and when pregnancy is discovered.