Now what the corpus lutein begins to do is it begins to release estrogen and it begins to release another hormone known as progesterone. So if we look at the following diagram, when the corpus luteum is formed, it begins to release estrogen. So we have an increase in estrogen and it also begins to release progesterone, shown here. 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.