Pre- and postnatal factors obtained from health records.

Collection of secondary data on an individual level, e.g. from official sources, may complement primary data from questionnaires and examinations in epidemiological field studies. Retrieval of individual-level secondary data thus represents an important step to constitute a comprehensive epidemiological database: secondary health data poten-tially represent an added value for the inference of causal relationships because they provide information without recall bias. In the IDEFICS/I.Family studies, health records of routine child visits reaching back to birth as well as medical records for the prenatal period were collected. Several stud-ies suggest that both intra-uterine and early infancy growth may influence the devel-opment of overweight during childhood, adolescence and even adulthood (Poston 2012). The IDEFICS/I.Family studies were conducted in different cultural settings using a standardised protocol that sometimes needed adaptation to local peculiarities. The lat-ter was the case for the documentation of routine child visits and maternity cards that varied across countries with regard to data sources and the type of information rec-orded. This chapter summarises the methodology of retrieval and harmonisation of secondary health data in the IDEFICS/I.Family studies and describes the differences and similar-ities of these records across countries.