Taking a multivitamin before and during early pregnancy has just been shown to be associated with a dramatic 57% reduction in the risk for miscarriage. If you are a woman who is sexually active and not sterile or infertile, whether you are taking birth control of not, you should, in my opinion, be taking a prenatal multivitamin every day.
More Information:
Supplemental vitamin recommendations for women who are pregnant, or are planning to become pregnant, are aimed mainly at reducing the risk of birth defects, Dr. Reem Hasan, at the University of North Carolina-Chapel Hill told Reuters Health.  
Their study provides evidence “that vitamins may reduce the risk of miscarriage as well,” Hasan said.
Between 2000 and 2008, Hasan’s team interviewed 4752 women during their first trimester of pregnancy to determine their use of prenatal vitamins and multi-vitamins.
Overall, 95 percent of the women reported using prenatal vitamins or multivitamins at some point during the first three months of pregnancy. About half the women reported taking vitamins prior to conception.
There were 524 miscarriages among the subjects. The researchers found that the risk for miscarriage was 57% lower among women who took vitamins, compared to those who did not.
This reduced risk was not altered by other factors including age, hormone use, the number of prior pregnancies, smoking status, race/ethnicity, educational level, and marital status, note Hasan and colleagues.
“Because miscarriage occurs very early in pregnancy, it is important for women of reproductive age, who may become pregnant, to eat a balanced diet and use vitamins.” Hasan advised.
However, the study was unable to account for dietary factors or healthy lifestyle behaviors, and did not differentiate between prenatal vitamin and multivitamin use. Therefore, Hasan’s group calls for further investigations into how these factors might alter miscarriage risk.
Nevertheless, this study, and others, back my contention that every woman of child-bearing age that is not sterile or infertile take a prenatal vitamin every day – whether you are on birth control or not.
The study was published in the American Journal of Epidemiology in June 2009. You can see the abstract here.