It is well established that many women are at risk of reduced folate status or even deficiency in their third trimester of pregnancy. This is probably due to an
Odd chain fatty acids
(Raised in plasma in vitamin B12 deficiency)
Methylmalonyl-CoA mutase (5'-deoxyadenosylcobalamin)
Figure 4 The role of vitamin B12 in the metabolism of propionates, odd chain fatty acids, and certain amino acids.
increased breakdown or catabolism of the vitamins associated with the rapid growth of the fetus/ placenta rather than the transfer of maternal folate to the fetus, which is quantitatively small. Rapid cell division would result in an increased flux through tetrahydro and dihydrofolate forms of the vitamin, which are known to be the two most chemically unstable forms of the vitamin. In many countries folic acid is given in the latter stages of pregnancy to protect against this risk of megaloblastic anemia, the emergence of which very much depends upon the mother entering the pregnancy with poor stores. These events in the third trimester should not to be confused with the more recent incontrovertible evidence that the maternal periconceptional ingestion of folic acid prevents the majority of cases of spina bifida and other neural tube defects, which take place within the first 4 weeks postconception.
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