What initiates the preparation of the endometrium for a potential pregnancy?

The preparation of the endometrium for a potential pregnancy is primarily initiated by estrogen. During the menstrual cycle, the ovaries produce estrogen, particularly during the follicular phase, which occurs after menstruation. As follicles develop, they secrete increasing levels of estrogen, resulting in the thickening of the endometrial lining. This process is crucial as a thicker, more vascular endometrium is essential for the implantation of a fertilized egg.

Estrogen not only promotes growth and proliferation of the endometrial cells but also enhances the receptivity of the endometrium for embryo implantation. Additionally, estrogen works in conjunction with other hormones later in the cycle but is fundamentally responsible for initiating the changes required for a possible pregnancy. This is why estrogen is the correct answer to the question regarding what triggers the preparation of the endometrium.