What event marks the start of a new menstrual cycle?

The beginning of a new menstrual cycle is marked by menstruation, which is the shedding of the uterine lining that occurs if fertilization has not taken place. Each menstrual cycle generally lasts about 28 days, although it can vary among individuals.

At the start of the menstrual cycle, typically on day one, the body expels the endometrial lining through the vagina, resulting in menstrual bleeding. This marks the transition from one menstrual cycle to the next, resetting hormonal levels and preparing the body for a potential new cycle of ovulation and possible pregnancy. After menstruation, the body begins to build up the uterine lining again, eventually leading up to ovulation. Once menstruation starts, the reproductive system is essentially preparing for the next opportunity for fertilization and implantation within the next cycle.