Doctors do not completely understand the true cause of migraine headaches. Some believe that migraines occur due to changes in the trigeminal nerve. Others believe chemical imbalances in the brain may cause the migraines. Serotonin levels drop during a migraine, which is one of the reasons people think brain chemicals play a role in migraine attacks. Doctors do know that the pain of migraines is due to the temporal artery enlarging, which cause the nerves that coil around the artery to release the chemicals causing the pain and many of the symptoms associated with a migraine headache.