"There are five things, whoever dies of any of them is a martyr. The one who is killed in the cause of Allah is a martyr; the one who dies of an abdominal complaint in the cause of Allah is a martyr; the one who dies of the plague in the cause of Allah is a martyr; and the woman who dies in childbirth in the cause of Allah is a martyr."
Sunan an-Nasa'i Book 25, Hadith 79