Because it is goes round 'in a circle' and the just about the same thing happens every month.
It goes through a follicular phase (days 1-14) and a luteal phase (days 14-28)
Body tempreture rises in the luteal phase
hormone level rises and peaks at approx. day 14 and progesterone levels rise considerably in the luteal phase
Within the menstruation cycle there is a uterine cycle which is the menses (blood comes out of the vagina) --> proliferative (blood starts to build up) --> secretory (full blood wall along the uterus)
Hope this sort of answered the question =L
