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. Now, what this describes is the fluctuation, the change in concentration of the levels of the hormone. So we have four hormones we have to consider. We have estrogen shown in green.