Folic acid supplementation has decreased neural tube defects in the USA and Canada by only about 35-45%, raising a suspicion in me that around half the population isn't able to fully utilize the folic acid that is now so plentiful in our food supply. To me, this, combined with our knowledge that half the population has difficulty turning folic acid into L-methylfolate, indicates that instead of folic acid, we should be consuming supplements of L-methylfolate.
Fortunately, it is now possible to purchase L-methylfolate and take that instead of folic acid. Merck sells it as Metafolin, and no prescription is required. A search of PubMed shows that scientists are starting to promote L-methylfolate's use instead of folic acid for women who could get pregnant. I buy L-methylfolate from Solgar on Amazon.com, and it is also available from various marketers of natural supplements, for people are starting to realize how frequently the MTHFR mutations appear to be correlated with a wide variety of health problems.