Trends in twinning rates in metropolitan Atlanta before and after folic acid fortification

Abstract

Objective: To examine whether trends in rates of twinning in metropolitan Atlanta increased after folic acid fortification.

Study design: Live births to residents of 5 metropolitan Atlanta counties during the period 1990-2001 were identified from the state of Georgia's vital records. Rates of twinning and rate ratios were computed for the periods before and after fortification with folic acid.

Results: Of the 510,000 singleton and twin births in metropolitan Atlanta during the study period, 7168 (1.43%) represented twin pregnancies. Overall, the rate of twinning increased 18% (P < .001)from the prefortification to postfortification period. As compared with the rate of twinning during the prefortification period, that during the postfortification period increased by 23% (OR 1.23, 95% CI 1.7, 1.28) among women over 30 years but showed no increase among women <30 years of age (OR 1.02, CI 0.94, 1.10). Among women >30, there was an increasing rate of twinning throughout the prefortification period (4.9%). There was no upward trend in twinning rates among women younger than 30 years prior to fortification.

Conclusion: Increasing trends of twinning were observed only in women older than 30 years, but these trends began prior to folic acid fortification and reached a plateau in recent years. Further elucidation of the possible relationship between folic acid and twinning will need to account for the use of fertility treatments by older women.