Data on age, sex, calendar period, maternal and paternal age at time of birth, and place of residence at time of birth were derived from the Danish Civil Registration System.11 Somatic comorbidities were assessed using the Charlson Comorbidity Index, which refers to a sum score built on 19 severe chronic diseases weighted from 1 to 6 corresponding to the severity of the disease.12 We further assessed the most common psychiatric comorbidities in persons with OCD and derived information on anxiety disorders (ICD-10 codes F40.00-F40.20, F41.00-F41.10, and F43.00-F43.10), depression (ICD-8 codes 296.x9, 298.09, 298.19, 300.49, and 301.19 and ICD-10 codes F32.00-F33.99, F34.10-F34.90, and F38.00-F39.99), and substance use disorders (ICD-8 codes 291.xx, 303.xx, 304.xx, 571.09, and 571.1x and ICD-10 codes F10-F16, F18, F19, I85, and K70) from the Danish Psychiatric Central Register9 and the Danish National Patient Register.10
