Sometimes, parents feel guilty when ADHD is diagnosed, but it's important to know that there is nothing you could have done differently to prevent it. ADHD isn't caused by bad parenting - although you might need above-average parenting skills to deal with its effects.
ADHD is defined by the 'core' signs of inattention (e.g. making careless mistakes, daydreaming or forgetfulness) and hyperactivity/impulsivity (e.g. fidgeting, squirming or excessive talking).