What causes your pregnant wife or gf to die after having the baby?

Serious question?: Everything that could cause her to die before she became pregnant, in addition to postpartum hemorrhage, pulmonary embolism, eclamptic seizure and stroke to name the big ones. Fortunately, the latter three are relatively uncommon in the US.

Traditionally: Historically, puerperal fever, or "childbed fever" was responsible for such deaths. In developed countries, more common causes are related to physiological stresses of labor and delivery, that is excessive bleeding, clotting, blood pressure.