Academic journal article Environmental Health Perspectives

"Birth Malformations and Other Adverse Perinatal Outcomes": Available Data Sources Pose a Dilemma

Academic journal article Environmental Health Perspectives

"Birth Malformations and Other Adverse Perinatal Outcomes": Available Data Sources Pose a Dilemma

Article excerpt

A recent paper (Schreinemachers 2003) highlights both the opportunities and pitfalls implicit in the use of national public use datasets. Schreinemachers (2003) used the national linked birth and infant death certificate files for calendar years 1995-1997 to study the prevalence and risk factors for adverse perinatal outcomes in high and low wheat-producing counties of four northern Great Plains states. Although Schreinemachers did not clearly state the null hypothesis, she attempted to test the assertion that in utero exposure to agricultural herbicides is associated with birth defects, preterm delivery, and small-for-gestational age (SGA) infants. Vital records for residents of 147 rural, agricultural counties were classified into low-wheat and high-wheat areas based on agricultural production statistics, using the median split method. What Schreinemachers (2003) described as an ecologic analysis is perhaps better categorized as a cross-sectional study with a dichotomous exposure variable classified by study-subject residence in a high or low wheat-producing county. This means that neither the exposure (agricultural herbicides) nor the outcome (birth defects) was adequately measured. The study results and discussion emphasized the statistical analyses of associations with congenital anomalies. Schreinemachers' (2003) Table 3 shows no effect of residence in a high-wheat county on preterm or SGA birth, and a modest increase in male infant mortality due to congenital anomalies. Results comparing low or very low birth weight, or overall fetal or infant mortality outcomes between the two groups of counties were not provided. Most of the results focused on overall and subcategories of birth defects (termed "developmental outcomes" in the title of Table 3); although some of the odds ratios are statistically significant, the author made no adjustment for multiple comparisons. Epidemiologists studying birth defects typically avoid analyses in which "births with any anomaly" is the dependent variable (p. 1262) because of the heterogeneity of the conditions thereby grouped together. Schreinemachers (2003) also neglected to include a map of the study counties, leaving the reader to wonder whether other physiographic, demographic, or economic factors might also influence the study findings.

If the primary study hypothesis is that preconceptional or antenatal exposure to agricultural herbicides increases the risk for birth defects across large geographic areas, it should be noted that this study used, at best, proxy measures for both dependent and independent variables. Because Schreinemachers (2003) provided no direct measures of exposure to herbicides, the reader must presume that differences in agricultural activity across counties correlated directly to individual exposures. Schreinemachers (2003) could have provided a reference to a study demonstrating that all residents of agricultural areas have similar levels of biomarkers of exposure to herbicides.

The most troubling aspect of this paper (Schreinemachers 2003) is its reliance on vital statistics for data on the occurrence of congenital anomalies among the birth events analyzed. Although numerous state programs have been developed in the past 15 years, Schreinemachers selected a study area for which statewide, population-based birth defects surveillance programs did not exist in any of the four states during the study period (National Birth Defects Prevention Network 2002). Her arguments concerning data quality are unconvincing and ultimately raise concerns that the study findings will be misinterpreted and misconstrued, and also will lead to similar analyses with methodologic flaws based on assumptions about the completeness, accuracy, reliability, and validity of vital statistics reporting of birth defects. It is not surprising that results in this study mirror that of Garry et al. (1996), given the use of birth certificate reporting of congenital anomalies to measure the outcome variable in both studies. …

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