Table 1.

List of variables and their categorization in the study

Variable	Categorization
Sex	Categorized into “boys” and “girls”.
Cohort	Categorized into 4 levels according to year of birth, “1973–1977”, “1978–1982”, “1983–1987”, and “1988–1993”.
Educational level	Categorized into 3 levels: “elementary school, 9–10 years”, “high school, 11–13 years”, and “graduate/postgraduate, ≥ 14 years”.
Parental country	Dichotomized into “both parents from Nordic
of origin	countries” and “one or both from non-Nordic countries”.
Marital status	Categorized into 3 levels: “married”, “unmarried”, and “divorced/widowed”.
Parental income	Income per year (SEK).
Smoking	Defined as the mother smoking on a daily basis. The amount of smoking was not known.
Perthes’	Children diagnosed with Perthes’ disease according to the National Patient Register and who had been given one of the following diagnoses. ICD-8: 722,10-722, and 11 722,19; ICD-9: 732B; and ICD-10: M911, M912, and M913. All other individuals were considered to be the control/comparison group.