In 1991-1994, only inpatients were included in the Danish Psychiatric
Central Register. From 1995, both inpatients and outpatients were included.
Information on diagnoses of tuberous sclerosis, Angelman syndrome, fragile
X syndrome, and congenital rubella, conditions associated with autism, was
obtained from the National Hospital Discharge Register.8 Information
on possible confounding factors was obtained from the Danish Civil Registration
System and the Danish Medical Birth Registry,9 as
follows: child's sex, child's place of birth (Copenhagen, Copenhagen suburbs,
area with ≥100 000 population, area with population of 10 000-99 999,
area with population of <10 000), birth weight (<2500, 2500-2999,
3000-3499, 3500-3999, ≥4000 g), 5-minute Apgar score (0-7, 8-9, 10), gestational
age (<37, 37-41, ≥42 weeks), mother's age at birth of child (<20,
20-24, 25-29, 30-34, 35-39, and ≥40 years), and mother's country of birth
(Danish or not).The percentage of missing values for the variables birth weight,
gestational age, 5-minute Apgar score, mother's country of birth, and child's
place of birth were 6.6%, 6.9%, 6.9%, 0.3%, and 0.03%, respectively.