AbstractEpidemiological studies of suspected human teratogens not infrequently lead to recurring weak or moderate associations (relative risks or odds ratios ranging from>1 to 3 for adverse effects and from 1/3 to<1 for protective effects) between specific defects and prenatal exposures. Examples of such associations include cigarette smoking and oral clefts (odds ratios between 1 and 2) and periconceptional multivitamin/folic acid supplementation and neural tube defects (odds ratios from 1/3 to 1). In this paper, we illustrate that low relative risk recurring in well‐designed studies may reflect underlying biologic mechanisms and should not be readily dismissed. Low relative risks could be the result of a combination of the following factors: 1) unmeasured confounding, 2) exposure misclassification (often related to the inability to pinpoint relevant dose and timing), 3) outcome misclassification (related to the etiologic heterogeneity of birth defects), 4) biologic interactions (related to teratogenic effects in population subgroups defined by genetic susceptibility or the presence of other exposures), and 5) differential prenatal survival (related to the combined impact of the exposure and the defect on prenatal survival). These issues can be addressed in epidemiologic studies by using biological markers of exposure and susceptibility, dysmorphologic evaluation of affected infants, subgroup analysis for etiologic heterogeneity, a search for biologic interactions, and the use of prospective cohort studies. Finally, low relative risks in the face of common exposures can reflect an important public health contribution of the exposure to the occurrence of the defect in the population. © 1992 Wiley‐Liss,