Migraines have been classified, based on how often they happen in a month: If a person has a headache for less than fifteen days, the migraine is called "episodic migraine" (EM). If it happens more than fifteen days, it is called "chronic migraine" (CM). Chronic means it happens over a long amount of time. Some people who start off getting episodic migraines may start to get "chronic migraines" later. Chronic migraine then may "revert" or go back to episodic migraine.