So let's take a look at the following diagrams that describe the menstrual cycle. And then let's describe the different steps of the menstrual cycle. So if we examine the diagram, notice the X axis describes our progression of days. 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.