And we have different types of estrogen. Now, what exactly is a function, what's the purpose of estrogen? Well, what estrogen basically does is it thickens, it increases the layer of the wall of the uterus, it increases the size of the endometrium and it prepares the endometrium for implantation by the zygote if fertilization actually takes place. So what that means is if that excel fuses with the sperm, eventually the zygote that is formed will make its way into the uterus and it will implant itself into that endometrium. And what estrogen does is it ensures that the endometrium is just the right layer, it's just the right thickness. So basically, we have this secondary follicle.