From anencephalie-info.org

What role does Folic Acid play in NTD prevention?

Folic Acid is a coenzyme. It plays an important role in many metabolisms. The developing unborn child needs it to grow cells, tissue and organs. During that phase, the folic acid requirements are higher than usual. As far back as 1976, scientists noticed that women who gave birth to NTD babies had low serum rates for folates and low vitamin levels in their red cells. In 1980, Professor Smithhells from Leeds (UK) was able to show that an additional intake of 0.4 mgs of Folic Acid before and at the start of a pregnancy significantly reduced NTD rates. This finding was confirmed by many other serious clinical studies using large numbers of women (over 250 000). The conclusive proof of the preventive effect of Folic Acid, even for women with no NTD history, came when a Hungarian study showed that, of the 2014 women who had taken additional Folic Acid, none developed an NTD, whereas, of the 2052 who had not, 6 cases were detected (Czeizel and Dudas, 1992). Recent research on NTD pathogenesis suggests that disorders linked to methioninesynthase activity could be one of the factors involved. This enzyme transforms homocysteine into methionine. To do so, it needs a methyle group which is provided by Folic Acid intake. If this transformation fails to take place, be it because of an enzyme anomaly or lack of Folic Acid, homocysteine levels increase. This would appear to prevent the closure of the Neural Tube. An additional intake of Folic Acid, coupled with an intake of vitamin B12 can rectify this anomaly which is in part connected with enzymes. Other studies have also shown that further anomalies such as heart malformations, lift and palate clefts and urethra malformations can be prevented by an additional intake of Folic Acid (Czeizel 1993, Antony 2000).

At what point and for how long should one take Folic Acid?

Folic Acid should be taken at least 4 weeks before a planned pregnancy.

NTDs are birth defects that occur between the 20th and the 28th day after conception, before most women know that they are pregnant. Because about half of all pregnancies are unplanned, it is important to include at least 0,4 milligrams of folic acid in every childbearing age woman's diet. However, for those women fortunate enough to confirm pregnancy very early on (i.e. 14 days post ovulation), and who have not been taking Folic Acid supplements prior to conception, you should begin immediately as there still may be some time for the supplement to benefit the developing embryo. Given that many pregnancies are not planned, the best way to prevent these birth defects is for all women of child-bearing age to ensure that they are taking enough Folic Acid everyday.

Women also need more Folic Acid during pregnancy and breast-feeding. Hence it is wise to continue to take pills after the critical moment when the Neural tube closes.

How much Folic Acid should one take?

0.4 mgs per day, in addition to the folates present in food. Multivitamin supplements containing less than 0.4 mgs of Folic Acid should not be taken in higher dosage in order to increase Folic Acid content. Too high an intake of other vitamins is harmful.
Prof. Andrew E. Czeizel presented a study in 2004 which showed that multivitamins containing 0,4 - 0,8mg of folic acid were more effective for the reduction of NTDs and other congenital abnormalities than high dose of folic acid.