So we begin at day zero and we end at day 28. And then the cycle repeats all over again. Now, what this describes is the increase in thickness of the endometrium found on the walls of the uterus. So we have an increase in the endometrium. Eventually we reach a point where the zygote does not implant because no fertilization took place. And then we have the process of menstruation, which basically decreases the thickness of that endometrium back to its basal of value.