European birth cohort studies on asthma and atopic diseases:: I.: Comparison of study designs -: a GA2LEN initiative

Background: The reasons for the rise in asthma and allergies remain unclear. To identify risk or protective factors, it is essential to carry out longitudinal epidemiological studies, preferably birth cohort studies. In Europe, several birth cohort studies on asthma and atopic diseases have been initiated over the last two decades. Aim: One of the work packages within the Global Allergy and Asthma European Network (GA(2)LEN) project was designed to identify and compare European birth cohorts on asthma and atopic diseases. The present review (part I) describes their objectives, study settings, recruitment process and follow-up rates. A subsequent review (part II) will compare outcome and exposure parameters. Methods: For each birth cohort, we collected detailed information regarding recruitment process, study setting, baseline data (pregnancy, birth, parents/siblings) as well as follow-up rates, outcome and exposure parameters at each time point. Results: We identified and assessed 18 European birth cohorts on asthma, allergic rhinitis and eczema. Six of these studies also focused on food allergies. The birth cohorts were mostly initiated in the 1990s with predominantly urban/metropolitan settings. Many studies were able to maintain high follow-up rates, even after five or more years. Conclusions: Due to the unique cooperation within the GA(2)LEN project a common database was established containing study characteristics of European birth cohorts on asthma and atopic diseases. This can be used as a basis for evaluating the possibility to pool data and perform meta-analyses, as well as to recommend criteria for conducting future birth cohorts.