Effects of folic acid fortification on twin gestation rates

OBJECTIVE: Previous studies have reported an increase in twinning of as much as 40% associated with folic acid-containing supplements, and folic acid fortification of enriched cereal grains was authorized in 1996. The purpose of this study was to investigate whether twinning rates have increased since that time. METHODS: We used United States birth and fetal death records to calculate twin gestation rates from 1990 through 2000. To eliminate the influence of fertility treatments, our analysis was limited to nulliparous women aged 16-19. We compared time trends in twin gestation rates before and after folic acid fortification in 1996. RESULTS: A total of 25,065 twin and 3,362,245 singleton pregnancies were included. Twin gestation rates were stable from January 1990 through December 1996, at 7.2 per 1,000, and then began a steady increase, averaging 2.4% (95% confidence interval 0.1-4.2%, P = .006) per year, which continued through 2000 and reached 8.2 per 1,000. This translates to 2 additional twin pregnancies per 10,000 gestations per year. Twin rates continued to increase well beyond 1998, when the maximal fortification effect on folate status had been reached. CONCLUSION: Although twin gestation rates in women not using fertility treatments increased after food fortification with folic acid, they rose by much less than the 40% rate previously reported; the observed pattern of increase in twin gestation rates is not consistent with a folic acid fortification effect.