Which hormone is primarily responsible for regulating menstruation?

Estrogen plays a crucial role in regulating the menstrual cycle. It is primarily responsible for the development of the ovarian follicles and the endometrial lining in preparation for potential implantation of a fertilized egg. Throughout the menstrual cycle, estrogen levels rise and fall, influencing various phases. In the follicular phase, rising estrogen levels help to stimulate the growth of the uterine lining and prepare for ovulation.

While progesterone is also important, especially during the luteal phase when it stabilizes the endometrium and maintains the uterine environment in case of pregnancy, it is the rise in estrogen that primarily drives the initial phases of the menstrual cycle. Therefore, estrogen's regulatory role in menstruation is fundamental to the cyclical nature of the menstrual cycle.