Study population
The source population included all children of the city of Espoo,
Finland, who were born between January 1, 1984, and March 31, 1990.
Espoo is an urban-suburban municipality with a population of over 300
000, located across the western border of Helsinki, the capital of
Finland. We took a random sample of children living in Espoo in 1991
drawn from the roster of Statistics Finland. The baseline study
population included a total of 2 568 children (response rate 80.3%)
whose parents gave informed consent and filled in the questionnaire.
Espoo is an urban-suburban municipality located across the western
border of Helsinki, the capital of Finland. In 1997, we conducted the
6-year follow-up survey of the cohort, with a follow-up rate of 77.3%,
and in 2010–2011, the 20-year follow-up (follow-up rate 63.2% of the
baseline population). The life-time home coordinates of the mothers and
children were retrieved from the Population Register Centre of Finland.
The study protocol was approved by the Ethics Committee of Oulu
University Hospital (Oulu, Finland).