And that's why we have an increasing level of these two hormones. And what progesterone does is it maintains the thickening of that endometrium. So it's estrogen that initiates the thickening process, but it's progesterone that maintains the thickening of that endometrium as shown in the following diagram. Now, in this particular case, estrogen created a positive feedback loop, but in this case, it will create a negative feedback loop as shown in this diagram. So at this particular point and by the way, notice that Ovulation and the LH surge took place on the 14th day mark. So that means after about two weeks, ovulation will take place as a result of that LH surge.