Getting too much folate may hide a B-12 deficiency, which is sometimes a problem for vegetarians.

http://www.babycenter.com/0_trying-to-conceive-five-changes-to-make-to-your-diet-now_3558.bc?page=2

For some women, there's an exception to this rule: Getting too much folate may hide a B-12 deficiency, which is sometimes a problem for vegetarians. Ask your doctor or midwife if you think you may be at risk.