Birth Cohort

A birth cohort is defined as a group of individuals born at or around the same time. These cohorts are defined in studies to examine the impact of genera-tion-specific factors that influence members of these cohorts. Differences in disease incidence are due to either exposures common to the birth cohort (e.g., early childhood disease that confers later immunity to diseases such as tuberculosis), the influence of early nurture (e.g., availability of child care for young children has a potential impact on later development), or events that have an age-specific impact later in life (e.g., adult women's consumption of calcium has a later impact on fractures).

An early example of a study that takes into account birth cohort effects is epidemiologist Wade Hampton Frost's examination of tuberculosis mortality trends by year of birth, an analysis that suggested that early childhood disease imparted later immunity to tuberculosis. Since then, birth cohort studies have been used to describe associations between birth year and later incidence of peptic ulcer, breast cancer trends, and reproductive health.

Large birth cohorts recruit a cross-section of individuals at or near birth and follow them over their life course. These are assembled for the purposes of studying exposures with long time lapse, and are potentially important for studying etiology of chronic diseases or other conditions that are impacted by longtime exposure. Current examples include the Early Childhood Longitudinal Study (United States), British Cohort Study (United Kingdom), and several birth cohort studies that are being undertaken in developing countries, including the Pelotas birth cohort study in Brazil and the Mexican Health and Aging Study.