Epidemiological studies show that the variance in ADHD phenotype has a strong genotypic (75 to 80%) and environmental (10 to 25%) contribution. The environmental contribution is mainly due to non-shared environmental factors rather than shared environmental factors.

Case/control epidemiological studies are unable to distinguish between shared and non-shared environmental factors. Our intra-familial design reduces the number of confounding factors, thus giving a more reliable picture of the environmental and genotypic contributions to the phenotype.

To compare incidence of Pregnancy, Labor, Delivery and Neonatal Complication (PLDNC) between children with ADHD and their unaffected siblings matched for age and gender.

Children with ADHD were recruited at the Douglas Mental Health University Institute The Kinney Medical and Gynecological Questionnaire and the McNeil-Sjöstrom Scale were used to assess incidence of PLDNC. The study had one hundred and sixteen sibling pairs (n=116) and a Mixed-Model Analysis of Variance (MMANOVA) was carried out.

Rank of birth and gender were significantly different between the two groups and these were taken as covariates. There was a significant difference in PLDNC between the two groups (F=4.49, df=1, p=.03). However, there was no significant interaction between the different stages of PLDNC and presence of ADHD.

PLDNC appear to be among the non-shared environmental factors implicated in ADHD. Children with ADHD have a higher prevalence of overall complications during pregnancy as compared to their non-affected siblings, and the period surrounding pregnancy might be of particular relevance to the aetiology of ADHD.