The association between folic acid deficiency and neural tube defects (NTDs) is well known. Most Gynecologists prescribe Tab. Folic Acid 5mg in the first trimester of pregnancy.

During many transformations of folate from one form to another, a proportion is accidentally converted to N5- methyl-THF, an inactive metabolite, the so-called ‘‘folate trap,’’ since there is no way for active N5, N10-THF to be regenerated except through a reaction for which a form of vitamin B12, methyl-B12, is a cofactor. Deficiency of B12 then produces a situation where more and more folate is trapped in an inactive form with no biochemical means

of elimination. The end result is failure to synthesize adequate DNA. Thereafter, folate no longer participates in its metabolic pathways, and megaloblastic anemia results. Vitamin B12 deficiency progressively leads to neurologic/ psychiatric abnormalities and increased excretion of folate in urine.

In individuals with vitamin B12 deficiency, consumption of folate may mask megaloblastic anemia, and when undetected, this progresses to subacute combined degeneration of the spinal cord. Folate also interferes with some antiepileptic or antifolate drugs. Therefore, consideration should be given to including B12 while prescribing folic acid to prevent NTDs. This is especially applicable to developing countries like India where the majority of women are vegetarians and have borderline levels of vitamin B12.

http://www.drnirajmahajan.com/files/documents/JPM-Folate-trap.pdf