Of Mice and Men...

[brewer]

Sure.

But you should know that NTDs develop in the first 3-4 weeks of pregnancy, before women even know they're pregnant after that termination is the only prevention for a NTD (and it is in fact required for pregnancies affected by anencephaly or encephalocele as the result of carrying the pregnancy to term is certain stillbirth).

So the only women who might be able to benefit are those like the OP who confirm pregnancy around 14 days after ovulation, as it gives them (maybe) six days before the NTD begins to develop. Maybe even less, we actually don't know. The only way to confirm a benefit now is observational studies, because we know that folate deficiency can cause NTDs we can't run experimental studies on humans as it would be unethical. And even if it wasn't unethical it'd be damn-near impossible to recruit participants that have just discovered they're pregnant 14 days after ovulation for your study. Logistically that's near impossible. By the time you found your participants, they'll already be more than 14 days past ovulation and goodbye any chances of getting meaningful results then.

The problem is a lot of literature uses the phrase "periconceptional". This is due to the fact that they've done observational studies (or even the experimental studies done in the early 90's), but there's no way you're going to find women that took a folic acid supplement and stopped the day that conception occurred without even knowing it. So the main problem is that the data cannot be split. But instead, the data can still show you when a woman started taking folic acid. So that's the only meaningful data - is how long before pregnancy did the women take the supplement and what were the results. And IIRC starting two weeks before conception has no benefit whatsoever as it is, let alone starting on the day of conception.

Anyway you want if from literature:

Lane, 2011: "Eighty-nine percent of women consumed supplements but only 31% took folic acid prior to conceiving. Hence, the vast majority are starting too late to prevent neural tube defects (NTDs)."

There are plenty more examples, but I'm not digging them all out.

Your statement was Quote: "Folic acid supplementation doesn't do anything once you've already conceived. " Then you only focus on NTD's. Do you really have that limited of a thought process? What about the rest of the pregnancy? Other fetal defects?

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3218540/

http://ajcn.nutrition.org/content/83/5/993.full