What is the primary action of estrogen in the female body?

The primary action of estrogen in the female body is to regulate the menstrual cycle and promote secondary sexual characteristics. Estrogen plays a crucial role in the development of female reproductive structures and the regulation of various phases of the menstrual cycle, particularly by influencing the proliferation of the uterine lining and preparing it for potential implantation of an embryo.

In addition to its role in the menstrual cycle, estrogen is responsible for the development of secondary sexual characteristics during puberty, such as breast development, widening of the hips, and the distribution of body fat in a typically feminine pattern. This hormone is fundamental to reproductive health and plays a significant role in maintaining various physiological processes in women.

The other options refer to functions not primarily associated with estrogen. For instance, while calcium regulation is important for various body functions, it is primarily influenced by hormones like parathyroid hormone and calcitonin. Muscle growth is more closely related to anabolic hormones, such as testosterone, rather than estrogen. Blood glucose regulation is mainly managed by insulin and glucagon from the pancreas. Therefore, the focus of estrogen as primarily responsible for reproductive and sexual health substantiates why the selected answer is correct.