The current research suggests too much folic acid (and other dietary supplements) during pregnancy may be related to an increased risk of allergies and asthma, and may even play a role in the dramatic increase in asthma prevalence during the past two decades. The U.S. Public Health service recommended in 1992 that all women of childbearing age consume 400 micrograms of folic acid daily to reduce their risk of birth defects of the spine and brain. In 1996 the U.S. Food and Drug Administration required that folic acid be added to specific flour, breads and other grains to prevent birth defects. Research has suggested that these measures have helped reduce birth defects.
Given the important role folic acid supplementation has played in prevention of birth defects, Drs. Schwartz and Hollingsworth do not advise any changes in folic acid supplementation, but do believe the issue is worth further investigation.
http://www.sciencedaily.com/releases/2008/09/080918192822.htm