Soon after the implementation of folic acid fortification in cereal grain products in 1998, several reports appeared in the medical literature showing a reduction in the prevalence of neural tube defects (NTDs). However, a number of investigators observed an ongoing background trend for decreased prevalence of NTDs that began before the implementation of fortification. It is therefore possible that the apparent reduction produced by folic acid fortification actually results from continuation of that trend. Previous studies obtained prevalence ratios consistent with a beneficial effect of fortification through use of a summary of prevalence before and after fortification. Their data, however, do not include an analysis of the magnitude and direction of the change in prevalence over time. This population-based study was designed to determine whether the trend for decline in the prevalence of NTDs observed before fortification (1989-1996) continued after fortification (1998-2003) in 8 central California counties. The study population included all live births, and stillbirths delivered in 8 central California counties from 1989 to 2003. The authors recorded the total number of NTD cases each year. Over the entire study period (1989-2003), 690 of the 886,985 total deliveries were NTD cases. This corresponds to an average prevalence of 77.8 cases per 100,000 deliveries. The authors compared the slopes of two regression lines, pre and postfortification, to provide an estimate of the annual change in NTD prevalence, and to compare the pre and postfortification changes in NTD prevalences over time. Analysis of the slope data for all NTDs combined showed that before fortification NTD prevalences were decreasing by 7.5 (slope: 27.5; 95% confidence interval [CI]: 212.4, 22.5) cases per 100,000 deliveries per year. After fortification, NTD prevalences were no longer decreasing. The postfortification slope exceeded the prefortification slope by 12.6 (95% CI: 2.6, 22.6) cases per 100,000 deliveries per year. These findings do not appear to result from low intake of folic acid during the postfortification period or differences in maternal race, ethnicity, or obesity. The authors conclude from these findings that the observed decline in NTD prevalence before fortification did not continue after fortification in selected central California counties. The data provide no evidence that fortification with folic acid reduced the prevalence of NTDs in this selected population.

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