What is the function of estrogen in the female reproductive system?

Estrogen plays a crucial role in the female reproductive system, particularly in regulating the menstrual cycle. It is primarily produced by the ovaries and its levels fluctuate throughout the cycle, influencing various physiological changes.

During the first half of the menstrual cycle, known as the follicular phase, estrogen levels rise as the follicles mature. This hormone helps to thicken the endometrial lining, preparing it for possible implantation of a fertilized egg. Additionally, estrogen works in conjunction with other hormones to trigger the luteinizing hormone surge, which is essential for ovulation.

While estrogen does contribute to the process of ovulation indirectly, its primary role in this context is more about regulating the overall menstrual cycle rather than directly causing ovulation. This regulation includes managing the timing of ovulation and the preparation of the uterine lining, making its influence on the menstrual cycle the most comprehensive answer.

Other options presented do not accurately reflect estrogen's roles. For instance, stimulating ovulation is not its sole purpose, and the production of testosterone or building muscle mass pertain to different hormonal functions or other systems in the body.