Sociodemographic Patterns in Spina Bifida Birth Prevalence Trends --- North Carolina, 1995--1999

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Prepared by
Robert E. Meyer, Ph.D.^1,3
Anna-Maria Siega-Riz, Ph.D.^{2,3

1}State Center for Health Statistics
Raleigh, North Carolina
²Department of Nutrition
³Department of Maternal and Child Health
University of North Carolina at Chapel Hill

Summary

Previous studies have documented a decline in neural tube defects (NTDs) in the United States after the addition of folic acid to enriched grain products and other folic acid initiatives. The decrease generally has been greater for spina bifida than for other NTDs. However, the extent to which the decline varies by maternal sociodemographic characteristics has not been examined. In this study, data from the North Carolina Birth Defects Monitoring Program, a statewide, population-based birth defect surveillance program, were used to assess the impact that folic acid public health initiatives have had on spina bifida rates among various sociodemographic subpopulations in North Carolina. This report covers data from 1995 through 1999. The overall prevalence of spina bifida decreased by 27.2% during 1995--1996 and 1998--1999 (p = 0.014). The magnitude of the decline varied considerably by sociodemographic characteristics of the mother. The decline was greatest among mothers who were aged >30 years (prevalence ratio [PR] = 0.53), who had more than a high school education (PR = 0.57), whose prenatal care was not paid by Medicaid (PR = 0.67), and who were non-Hispanic white (PR = 0.72). Geographically, the decrease in the western and Piedmont regions of the state was almost threefold that occurring in the eastern region. The decline in spina bifida after fortification varied considerably by sociodemographic subpopulations. More effort is needed to target folic acid education programs at disadvantaged populations.

Introduction

Previous studies have demonstrated a decline in the birth prevalence of spina bifida after the addition of folic acid to enriched grain products and the initiation of folic acid education programs during the 1990s. An analysis of birth certificate data in the United States found that the rate of spina bifida declined 16% during 1991--2000 (1). A subsequent study using U.S. birth certificate data documented a 23% decline in spina bifida after fortification (2), and a recent analysis of data from 24 state birth defects surveillance programs also demonstrated a decline in spina bifida prevalence since the mid-1990s (3). Although the findings of these studies demonstrate that spina bifida has declined in the years following fortification, the extent to which this decline varies by sociodemographic characteristics has not been assessed. This report describes the association between maternal sociodemographic factors and recent trends in spina bifida in North Carolina. This information can be helpful for assessing the effects that folic acid initiatives have had on various segments of the population and for identifying subgroups for which targeted educational activities are most needed.

Methods

Case Definition and Ascertainment

Cases of spina bifida were identified from the North Carolina Birth Defects Monitoring Program (NCBDMP). The NCBDMP is a statewide, population-based surveillance system that collects information on congenital malformations diagnosed within the first year of life among North Carolina resident liveborn infants and among stillborn infants aged >20 weeks' gestation. The surveillance system covers a birth population of >110,000 deliveries annually. Beginning with births occurring in 1995, the NCBDMP initiated an enhanced system for ascertaining infants with neural tube defects (NTDs). Trained case abstractors conducted routine visits at the state's 12 tertiary hospitals to review medical records and abstract information about infants suspected of having NTDs or other birth defects. Suspected cases were identified through reviewing hospital disease indexes and through genetics and prenatal diagnosis logs in selected hospitals. Data sources used to identify cases at non-tertiary hospitals include hospital discharge data, vital statistics, and Medicaid paid claims files. All suspected NTD cases were confirmed through chart review.

Cases of spina bifida were defined according to International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes 741.00--741.93 (4) and included those affecting all resident liveborn infants and stillborn infants aged >20 weeks' gestation delivered during 1995--1999. A total of 299 spina bifida cases were identified during the 5-year study period, resulting in 279 live births and 20 fetal deaths.

Data Analysis

Cases were matched with the North Carolina composite matched birth files for 1995--1999. This database contains information extracted from vital statistics files (matched birth and infant death certificates), Medicaid paid claims, and other health services-related data for all North Carolina resident births. The prevalence of spina bifida (number of cases per 10,000 births) for each year was calculated overall and for the following maternal characteristics: age, race, education, Medicaid status (as defined by whether the delivery was paid for by Medicaid), and geographic area of residence.

Poisson regression was used to examine the overall trend in the spina bifida prevalence during 1995--1999. This method is appropriate for modeling disease incidence data in which the event (numerator) is relatively rare in proportion to the total population at risk (5). A Poisson log-linear model was fitted to the data by using SAS PROC GENMOD (6), modeling the change in rates as a function of year. To assess whether the change in prevalence varied according to maternal characteristics, the prevalence for the most recent 2-year period (1998--1999) was compared with that of the earliest period (1995--1996) for each of the previously described sociodemographic variables. Prevalence ratios (PR) and 95% confidence intervals (CI) were computed to compare the two time periods.

Results

The number of spina bifida cases in North Carolina declined from 63 in 1995 to 48 in 1999, despite the 12% increase in the total number of live births in the state during this period. During 1995--1999, the overall prevalence of spina bifida declined by 32% (p = 0.015, based on Poisson regression), from 6.20/10,000 live births in 1995 to 4.22/10,000 in 1999 (Figure).

Comparing the earliest and most recent 2-year periods (1995--1996 and 1998--1999), the overall spina bifida prevalence declined by 27.2% (PR = 0.73, 95% CI = 0.56--0.94). However, the magnitude of the decrease differed substantially according to maternal characteristics (Table). The greatest declines were seen among mothers in the older age groups, especially women aged >30 years, in which the prevalence decreased by 47.0% (PR = 0.53, 95% CI = 0.33--0.84). In contrast, among women aged <25 years, the prevalence remained virtually unchanged (PR = 0.99, 95% CI = 0.67--1.45). The spina bifida prevalence for women with more than a high school education declined by 43.2% (PR = 0.57), which was about 2.5 times the amount of decline observed among women with a high school education or less. Among women whose prenatal care was not paid by Medicaid, the prevalence declined by about one third (PR = 0.67), compared with an 18.5% decrease for women receiving Medicaid. Differences in the decrease in spina bifida by race were less marked, although the decline among white non-Hispanic mothers was slightly greater compared with the decline among minority women (PR = 0.72 and 0.76, respectively). Geographic differences also were observed in the decline of spina bifida; decreases in western North Carolina and in the Piedmont region were nearly threefold those experienced in the East.

Discussion

The decrease in the rate of spina bifida since the mid-1990s is consistent with previous reports that have demonstrated similar declines in the United States (1--3). However, this study reveals that the reduction in spina bifida in North Carolina was not uniform across the population, but varied by geographic region as well as by maternal sociodemographic characteristics. The decline was generally greater among women who tend to be of higher socioeconomic status (i.e., those who are older, have higher education, and are not receiving Medicaid. The decrease was also considerably greater in the western and Piedmont regions of the state compared with the eastern coastal plain, a pattern that reflects the sociodemographic differences among these regions. In addition, western North Carolina was the first area of the state to initiate a well-coordinated and intensive folic acid education program; this effort may have contributed to the dramatic decline observed in that region.

Much of the recent decline in NTDs in the United States has been attributed to the mandatory fortification program rather than to increased intake of multivitamins. Data from the March of Dimes/Gallup surveys indicate that the percentage of women aged 18--44 years who take daily multivitamins has increased only slightly since 1995 (7). In addition, preliminary data from the National Health and Nutrition Examination Survey (NHANES) IV indicate that supplement use among women of childbearing age was similar to that reported among women participating in the NHANES III survey; yet, blood folate levels increased significantly during the 5 years between the two surveys (8). As indicated by the present study, however, the fortification program in the United States may not have benefited all segments of the population equally. One explanation is that sociodemographic differences in food consumption patterns and multivitamin intake may be mediating the effect that fortification is having in various subpopulations.

Some nationally representative studies have documented differences in consumption patterns of enriched grain products by sociodemographic characteristics. For example, persons with higher income, those with more education, and non-Hispanic whites tend to consume more cereals (9--12). These differences in cereal consumption are noteworthy because of the now widespread availability of ready-to-eat breakfast cereals that are fortified at 100% of the recommended daily value. Furthermore, total folate intakes are higher among non-Hispanic white and black women in the middle and upper socioeconomic classes compared with the lowest (US Department of Agriculture. Continuing Survey of Food Intake by Individuals, 1994--1996, unpublished data). Although more studies are needed to determine whether these food consumption patterns persist in the postfortification era, the data that are available correlate with the sociodemographic trends in spina bifida reported in this study.

Like dietary patterns, multivitamin use also differs by sociodemographic characteristics (13--15). Several studies demonstrate that older, higher income, non-Medicaid, and better educated women are more likely to be taking supplements. The greatest declines in spina bifida have occurred among infants born to women of these same sociodemographic groups.

Folic acid fortification might be having a greater effect in reducing NTD risk among those women whose nutritional status is already near an optimal level, e.g., women who are more likely to consume ready-to-eat cereals, to take multivitamins on an occasional basis, and to maintain a more well balanced diet. The current fortification level has been estimated to increase the average woman's consumption of folic acid by about 100 µg per day (16). Although this amount is only about one fourth of the total intake recommended by CDC for birth defects prevention, it may be sufficient for women whose baseline folates were already at or above the prefortification median levels. For these women, fortification may have provided sufficient additional folic acid in their diets to raise their folate levels to the optimal threshold needed to prevent birth defects. Such women tend to be from more affluent sociodemographic populations. In contrast, among women from more disadvantaged groups who tend to be in poorer nutritional status and are less likely to take any multivitamins, the current fortification level might be considerably less than the amount needed for full birth defects prevention.

The possibility that the recent decline in spina bifida is only part of a secular trend in the prevalence of NTDs cannot be ruled out. Throughout much of the world, the rate of NTDs has been declining during the past several decades (17), and to some extent, the decrease seen in recent years can be viewed as a continuation of that trend. In this context, gauging the extent to which folic acid interventions may have contributed to the recent decline is difficult. Nevertheless, the increase in blood folate levels in the United States, combined with evidence of concomitant decrease in NTDs from multiple studies, supports the view that folic acid initiatives are contributing to the prevention of NTDs.

The extent to which the recent decline in spina bifida and other NTDs may be attributed to mandatory fortification, increased multivitamin intake, or increased availability of fully fortified, ready-to-eat cereals cannot be assessed with the data available. Most likely, all these factors combined have led to the dramatic improvements seen in recent years and help explain why certain segments of the population have benefited more than others.

Additional studies using data from population-based birth defect surveillance programs are needed to confirm the results of this study. The present findings indicate that substantial potential still exists for further reductions in spina bifida, particularly among minorities and women of lower socioeconomic status. With the effects of the fortification program probably having already been realized, sustaining the decline will require more aggressive efforts to encourage minority women of childbearing age, as well as those from lower socioeconomic groups, to take daily multivitamins containing 400 µg of folic acid in addition to eating a well-balanced, folate-rich diet.

Acknowledgment

Project support was provided through a cooperative agreement (U50/CCU416075) with CDC and through a grant from the North Carolina Chapter of the March of Dimes. The authors thank Paul Buescher, Donald Mattison, Godfrey Oakley, and Andrew Olshan for their comments and advice on an earlier version of the manuscript.

References

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