Norwegian researchers exploring facial cleft and cleft lip prevention, concluded that a mother's supplementation of 400 mcg or more of folic acid during early pregnancy reduced the risk of cleft lip by about one-third. Norway has one of the highest rates of facial clefts in Europe; and it's noteworthy that fortification of foods with folic acid is not allowed in Norway.
Folic acid (a B vitamin also called folate) is in foods such as leafy green vegetables, fruits, dried beans, peas and nuts. In the United States, certain foods are also enriched with folic acid. The United States National Women's Health Information Center encourages all women of childbearing age to consume 400 mcg per day because folic acid is known to help reduce the risk of neural tube defects in early pregnancy.
Wilcox, A. et al. "Folic Acid Supplements and Risk of Facial Clefts: National Population Based Case-Control Study," BMJ, 2007;334:464;originially published online Jan. 26, 2007; doi:10.1136/bmj.39079.618287.ob
"Folic Acid" http://womenshealth.gov/faq/easyread/folic-etr.htm