Which hormone is primarily responsible for regulating the menstrual cycle?

Estrogen is primarily responsible for regulating the menstrual cycle due to its critical role in the development of the ovarian follicles and the preparation of the endometrium for potential implantation of a fertilized egg. During the follicular phase of the menstrual cycle, estrogen levels rise as the follicles mature, stimulating the thickening of the uterine lining. This hormone also triggers the surge in luteinizing hormone (LH), which is essential for ovulation.

The menstrual cycle consists of several phases: the follicular phase, ovulation, the luteal phase, and menstruation. Estrogen levels fluctuate throughout these phases, influencing various physiological changes. For instance, after ovulation, progesterone levels increase to maintain the uterine lining, but it is estrogen that primarily coordinates the initial stages of the cycle. Thus, while progesterone is important for the latter part of the cycle, it is estrogen's regulation of follicular development and uterine preparation that makes it the primary hormone in governing the menstrual cycle.