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

Estrogen plays a critical role in the female reproductive system, primarily through its function in preparing the endometrium for implantation. During the menstrual cycle, after ovulation, estrogen levels rise and stimulate the thickening and vascularization of the endometrial lining. This preparation is essential for the potential implantation of a fertilized egg. The increased blood supply and nutrient availability in the endometrium create a conducive environment for a developing embryo.

This function of estrogen is vital for reproductive success, as a well-prepared endometrium enhances the chances of implantation and subsequent fetal development. Other options, while relevant to different biological processes, do not describe the primary role of estrogen in this specific context. For example, the stimulation of milk production is primarily driven by prolactin, while sperm motility is influenced by testosterone and other factors. Regulation of water balance is mainly managed by hormones like aldosterone and antidiuretic hormone, not estrogen.