Does the use of folic acid increase the risk of twinning?

The increased rates of twinning and the increased consumption of folic acid are two parallel processes described in recent years. However, the possible association between an increased incidence of twin pregnancies and periconceptional folic acid supplementation remains controversial. Whereas the data indicating that when folic acid levels are increased to the range required for reducing the incidence of neural tube defects, the data pertaining to the increase in twinning rate are significantly incoherent and are flawed by serious confounders, such as fertility treatments and maternal age. The only plausible theory connecting methyltetrahydrofolate reductase mutation, folic acid, and twinning has not been confirmed. In addition, temporal and dose-effect relationships have not been established. Because of the lack of coherence, questionable plausibility, and no clear dose-effect and temporal relationships, a cause (folic acid supplementation) and effect (increased twinning rate) association cannot be, at present, accepted. Thus, the established policy of peri-conceptional folic acid supplementation to reduce the incidence of neural tube defects should be continued.