First of all, the reason supplementation was ever suggested was because it was thought to prevent neural tube defects like spina bifida (where the spinal column doesn't properly close in the womb). It is now recommended that all women who may become pregnant take 400mcg of folic acid every day, since neural tube defects occur early in pregnancy. This level has reduced birth defects by 41% in populations like the US.
It is important to realize, however, that folic acid is the synthetic form of folate, which is found in foods. Folate is what our bodies are designed to use, and folate and folic acid are not exactly the same. Even your prenatal vitamin probably contains folic acid and not folate! Since women could not be trusted to get enough folate from their diets folic acid began to be added to cereals, breads, and pasta.
Dark, leafy greens, beans and liver have the highest concentrations of folate. While these foods were eaten a century ago, our way of eating has changed. We rarely eat greens, especially in large quantities. Liver is almost unheard of! Beans are mostly an occasional side dish. Folate is water-soluble, so remember that if you're boiling your greens you're throwing the folate away. Another way to get folate is to use that water-solubility to your advantage. Either drink the little bit of water left after cooking your greens in a small amount of water, or make an strong infusion of a folate-rich herb like stinging nettle. This will provide you with easy folate and is safe in pregnancy.
Just adding folic acid to all our foods is not as helpful as it first sounds. There are some people born with a genetic mutation called the MTHFR mutation that makes it impossible for them to turn folic acid into the kind of folate their bodies use. Since we are just learning about this and the symptoms are widely varied this mutation is often not found. Could this be the reason folic acid supplementation doesn't prevent birth defects, but only reduces them? Some people with a specific form of the MTHFR mutation need to avoid folic acid entirely and only take folate since they lack the ability to convert it.
Another consideration is that folate is part of the B-vitamin complex, so it works best in conjuction with other nutrients, especially B12 and iron. Just adding high levels of one thing is not nearly as effective (and could sometimes be harmful) as finding all the nutrients together in healthy foods.
I hope the message is clear that there is no easy way to "make up" for an unhealthy diet by just taking a prenatal vitamin 3 times a day. Yes, even if it contains folic acid. Unfortunately, we have no way to supplement ourselves into good health and still eat poorly.