Why was the cohort set up?
The developmental origin of health and disease hypothesis and the interest in life course epidemiology have fuelled the creation of numerous birth cohorts during the past three decades.1,2 The largest Danish and Norwegian birth cohorts each includes in the range of 100 000 pregnancies3,4 but still have limited statistical power for the study of rare exposures and outcomes. This hampers, not least, studies of specific occupational exposures in which prevalence commonly lies in the range of 3–5%.5 With the development of an increasing number of validated job exposure matrices (JEMs), covering numerous types of occupational exposures,5 it has become attractive to leverage on the high-quality Danish health and labour market registries with national coverage to build a national register-based occupational cohort with data on the offspring. In addition to the large number of individuals, which enables studies of less frequent exposures and outcomes, the cohort provides data over several decades that make time-trend studies as well as studies of effects on offspring from parental early-life exposures feasible. Moreover, it will become possible to design advanced twin, sibling and half-sibling designs to minimize bias.