Why do i get migraines twice a year?

I've been getting progressively worse migraines twice a year for over 10 years. They usually start march and end in April and then start again mid sept to mid Nov. Once or twice over the years they haven't turned up and im clueless as to why as i wasn't doing anything different than normal the times they missed a year. I had acupuncture out of desperation one year when it was really bad and to my total surprise i didn't have another migraine for 1 1/2 years. Would this indicate the cause of my pain is more likely to be something physical rather than chemical? im confused. and in pain >.<