What triggers menstruation to occur?

Progesterone drop: The trigger for menstruation is a drop in your body's progesterone level. When you ovulate, the egg ruptures out of a fluid filled follicle on the ovary. It leaves behind a biologic timer--a temporary gland called the corpus luteum. This is a temporary gland that produces progesterone for about 14 days and then goes away. When it goes away, your progesterone drops and you have a period.