Use of folic acid supplementation and risk for dizygotic twinning

BACKGROUND: An increase in dizygotic twinning rate after folic acid supplementation has previously been described, but in a recent study from China, no such effect was seen. AIMS: To further investigate the association between use of folic acid supplement and dizygotic twinning. STUDY DESIGN: Using the Swedish Medical Birth Registry, the rate of dizygotic twinning among infants of women who reported the use of folic acid was studied, taking a number of confounders into consideration. Comparisons were made with all women recorded in the register. OUTCOME MEASURE: Unlike-sexed twin pairs were used as representatives of dizygotic twinning. RESULTS: A number of confounders for the association between folic acid use and twinning were identified and taken into consideration. Women who were immigrants (who used less folic acid and had slightly lower twinning rates than women born in Sweden) and women who reported subfertility problems or treatment for subfertility (who had a high use of folic acid and a high twinning rate) were excluded, and adjustment was then made for year of birth, maternal age, parity, and smoking in early pregnancy. Folic acid use and twinning risk increased with maternal age and decreased with parity. Smokers used less folic acid than nonsmokers. Concomitant use of other drugs was no important confounder except for those used at subfertility. The odds ratio (OR) for dizygotic twinning after folic acid supplementation was then 1.71 (95%CI=1.21-2.42) and for the years 2000-2001 even 2.09 (95%CI=1.39-3.12). CONCLUSION: There is an increase in twinning rate after folic acid supplementation in a Western population which should be taken into consideration when folic acid supplementation or food fortification is recommended.