Folic acid probably doesn't increase dizygotic twinning after all

Although there's research to suggest that folic acid increases the risk of twins, the data are seriously flawed because they fail to take into account the large number of women in the study that had to undergo IVF.

Since women who have IVF are about 20 times more likely than those who do not have twins, the investigators were quick to point out that unless the status of IVF and ovarian stimulating drug use is known for virtually all women in a study, a false association between twinning and any drug or vitamin used in combination with IVF is likely to occur.

Berry RJ, Kihlberg R, Devine O. Impact of misclassification of in vitro fertilization in studies of folic acid and twinning: modelling using population based Swedish vital records. BMJ. 2005;330:815.