Preventing birth defects is a relatively new and rapidly expanding field. March of Dimes (1-888-MODIMES) offers some good information. There are also many web sites that can be found by typing in birth defects (or heart defects, etc.) into a search engine.

The single most important concept is timing. Birth defects happen very early in pregnancy, generally in the first 2 months, and often before a mother-to-be even knows she is pregnant. The time to think about preventing birth defects is before conception. Most of the causes of birth defects are still unknown.

Probably something like a fourth of birth defects are caused by genetic problems. In 1987 Dr. Mark Bogart discovered the screening method now widely used to detect and prevent Down syndrome and neural tube defects (US patent 4,874,693). New genetic markers are being discovered almost daily. Roughly another fourth of birth defects are caused by various infections and toxins. Some of these may be prevented with information, and in addition, there are now what appear to be relatively safe treatments to block the effects of many agents that cause birth defects (Geber, 1992, US patent 5,100,878). The remaining half of birth defects are probably caused or made worse by nutritional problems.

Folic acid is the only nutrient that has been convincingly proven to prevent human birth defects. The state of the art is to take a multi-vitamin that contains folic acid, starting at least a month before conception. There have been several good clinical trials of this technique, showing it prevents about half of all birth defects, especially neural tube defects. (There is a good article and editorial in the Dec 24, 1992 The New England Journal of Medicine.) 

While folic acid clearly deserves the spotlight, there are at least 7 other nutrients that deserve mention. For each of these someone has found at least one small reason to make me think the nutrient might be able to prevent some birth defects.

Folic acid

Vitamin A (beta carotene is equivalent and safer in early pregnancy)

Vitamin B12

Zinc

Manganese

Selenium

Chromium

Fluorine

(Another set of 4 may someday be on this list: Vitamin D, Copper, Methionine, and Lysine.)

The top 4 nutrients above are in most regular vitamin products, and almost all prenatal vitamin products. The next 3 have recently been appearing in more and more of these products, but youll probably have to look to find them. Fluorine (usually called fluoride) is the one that is the least likely to be there, and it is the subject of the remainder of this web site. For each of these nutrients, I doubt anything more than about ½ the RDA is needed, and I do not suggest taking large doses of any nutrient during early pregnancy.