Gestational age and HKD
Compared with children born at term, children with gestational ages between 34 and 36 completed weeks had an 80% increased risk of HKD, and children with gestational ages below 34 completed weeks had a threefold increased risk (unadjusted results) (table 1).
Birth weight and HKD
Among children born at 37 completed weeks of gestation, the mean birth weight was lower among cases compared with controls (unadjusted difference −98 g; 95% CI −134 to −61). Children born at term with birth weights between 1500 and 2499 g had more than a twofold increased risk of HKD compared with children born at term with birth weights above 2999 g, whereas children with birth weights between 2500 and 2999 g had a 70% increased risk (unadjusted results) (table 1).
Gender and age
Of the 834 children with HKD, 750 (90%) were boys. The increased risk of HKD was basically the same in both genders, but analyses on girls only yielded very wide confidence intervals as most of the cases were boys. The age of the children at the time of diagnosis varied between 2 and 18 years (median 8.8; interquartile range 3).
Social factors, previous admissions, family history of psychopathology
Single parent families, disadvantageous social factors, and young age of the parents were associated with an increased risk of HKD in the offspring; the effect of the income level of the father was generally higher than for the mother (table 2).
Previous psychiatric admissions and contact as outpatients of the cases and the controls (RR 21.7, 95% CI 17.0 to 27.5) and psychopathology in the immediate family, measured as admissions to psychiatric hospitals or departments or as outpatient contacts for the mother (RR 2.6, 95% CI 2.1 to 3.3), the father (RR 2.1, 95% CI 1.7 to 2.7), or siblings (RR 3.9, 95% CI 3.0 to 5.1) increased the risk of HKD.
Adjustments for social factors, history of psychiatric disorders in the parents and siblings, and parental age did not change the results substantially (table 3). The small changes in the risk estimates were due to a joint effect from all the variables under study and not accounted for by a single variable. When the results on birth weight in table 3 were adjusted for gestational age in weeks, the results were also unchanged (adjusted difference −94 g; 95% CI −143 to −45). For both exposures under study, a dose‐response relation was present
Taking into account previous admissions and outpatient contacts and the time period since the last admission before the diagnosis, the results remained unchanged (data not shown). The results were essentially unchanged after restriction to children with parents or siblings without psychiatric admissions and contact as outpatients, thereby excluding 184 cases and their controls (data not shown).
Conduct disorder and other comorbidity
Excluding cases and their controls with conduct disorders either as main diagnosis (F90.1; 194 cases) or as subsidiary diagnoses (F90.1, 91.1, F91.3, F91.9; 13 cases) also failed to change the results. Exclusion of the 206 children with HKD (and their matched controls) with comorbid disorders other than specific developmental disorders of speech, language, scholastic skills, or motor function recorded as subsidiary diagnoses did not change the results.
Excluding children of parents with a history of mental disorders, children with hyperkinetic conduct disorder (F90.10), and children with other comorbid disorders except for specific developmental disorders (F80.0–F89.3) did not change the results (table 3). The mean birth weight remained lower among cases compared with controls.
Maternal smoking during pregnancy and gestational age
Among children born between 1991 and 1994 (n
170), information on maternal smoking status was available (smoker, non‐smoker).18
When the analysis of preterm delivery and HKD was performed among non‐smokers (n
65), the risk was still increased (RR 4.1, 95% CI 1.4 to 11.8).