The zygote is the combination of the sperm and our egg. This process of combining is known as fertilization. Now, the second purpose of estrogen is basically to promote the development of female secondary sex characteristics such as our enlargement of breasts, as well as the growth of body hair in different parts of the body. It also is responsible for increasing the muscle mass and bone mass of women. Now, let's move on to the other type of female sex hormone known as progesterone. So progesterone is also a steroid hormone that means it's formed from cholesterol and it is lipid soluble.