Pteroylglutamic acid is the systematic chemical name for folic acid, the synthetic form of vitamin B9 used in supplements and fortified foods. Its structure joins three parts: a pteridine ring, para-aminobenzoic acid, and glutamic acid. In the body it is reduced and methylated to tetrahydrofolate forms that carry one-carbon units in the synthesis of DNA, the metabolism of amino acids, and the formation of red blood cells. Because demand for new DNA is highest during rapid cell division, adequate folate is critical in pregnancy, where deficiency raises the risk of neural tube defects, and in infancy. Folic acid is also the form added to flour in mandatory fortification programmes adopted by many countries to protect women of childbearing age.