Which of the following best describes the main role of estrogen in the female reproductive system?

• A. It inhibits ovulation
• B. It establishes and protects the endometrium
• C. It promotes lactation
• D. It triggers menstruation

Answer: (B)

Estrogen plays a crucial role in establishing and maintaining the endometrium, or the lining of the uterus. During the menstrual cycle, estrogen is secreted primarily by the developing follicles in the ovaries during the follicular phase. This hormone stimulates the proliferation of endometrial cells, enhancing the thickness and vascularization of the endometrium, which prepares it for potential implantation of a fertilized egg. Thus, its main role can be seen as establishing and protecting the endometrium, ensuring that the uterine environment is suitable for pregnancy.

In contrast, while estrogen does play indirect roles in other processes, such as the overall regulation of the menstrual cycle, it does not inhibit ovulation directly, promote lactation, or trigger menstruation itself. Ovulation is primarily triggered by a surge in luteinizing hormone (LH), lactation is mainly driven by prolactin and the interactions of other hormones postpartum, and menstruation typically occurs when estrogen and progesterone levels drop, leading to the shedding of the endometrium rather than being directly initiated by estrogen.