Epidemiologic approaches to identifying environmental causes of birth defects

Epidemiology can be used to elucidate environmental causes of birth defects. This paper discusses 1) different types of environmental causes; 2) the difficulties in comparing the prevalence of birth defects between populations, including the need for a population base and the implications of prenatal diagnosis; 3) the main study designs for observational epidemiological studies and the various sources of bias; 4) how statistical power can be increased by meta-analysis or multicentric studies, and improved grouping of birth defects into etiologically more homogeneous subgroups; 5) the distinction between association and causation; 6) the interpretation of clusters in time and space in relation to local environmental causes; and 7) the potential of genetic epidemiology to help elucidate environmental causes. While further research continues into the environmental causes of birth defects, the epidemiologic evidence base for policy making and clinical practice is poor in many areas. The epidemiologic approach is important not only to elucidate environmental causes but also to assess the implementation of existing research into policy and practice for the prevention of birth defects.