Objectives: To study the factor of birth weight below 2,500 grams and Attention Deficit Hyperactivity Disorder (ADHD) and to identify the factors that are associated with ADHD.
Material and Method: A retrospective, hospital – based, case control study was conducted with 122 ADHD cases from the child and adolescent psychiatric outpatient unit at Siriraj Hospital, 119 nonADHD from students who have been assessed from Child Behavior Checklist (CBCL) and Conners Teacher Rating Scale, they are identified as normal. Mothers of both groups have been asked to fill a self report questionnaire to evaluate demographic data, prenatal data, factors effected pregnancy both physical and mental illness, post natal period, birth weight, underlying disease, physical illness and family history of ADHD. Multiple logistic regression analysis was used to determine the adjust effect of all factors.
Results: The number of ADHD cases who had a birth weight below 2,500 grams was 3.6 times the number of control cases who had a birth weight below 2,500 grams. It was statistically significant ( p = 0.03). Factors associated with ADHD were pregnancy complication (p value < 0.05, OR = 4.17, 95%CI [1.66, 14.02]), emotional distress during pregnancy (p value <0.05, OR = 2.99, 95%CI [1.43, 5.40]), postnatal complication
(p value < 0.05, OR = 3.26, 95%CI [1.56, 6.41] and a family history of ADHD (p value < 0.05, OR = 3.6, 95%CI [1.65, 8.11].
Conclusion: From the present study, the birth weight below 2,500 grams, pregnancy complication, emotional distress during pregnancy, neonatal complication and a family history of ADHD may be the risk factors of ADHD.

Keywords: ADHD, Birth weight, Perinatal complication, Pregnancy complication