The menstrual cycle begins with the period, or menses, defined by the shedding of the uterine lining. This event is a direct consequence of a dramatic shift in reproductive hormone levels. The cycle involves four primary hormones: estrogen, progesterone, Follicle-Stimulating Hormone (FSH), and Luteinizing Hormone (LH). The start of the period is characterized by the lowest levels of the steroid hormones (estrogen and progesterone) and the rise of the pituitary hormones (FSH and LH). Understanding these shifts clarifies the physical experience of menstruation and the preparation for the cycle ahead.
The Decline of Estrogen and Progesterone
The period results directly from a sudden, steep fall in progesterone and estrogen levels. These hormones peak during the preceding luteal phase, preparing the uterus for potential pregnancy after ovulation. If fertilization does not occur, the corpus luteum—the structure producing these hormones—breaks down.
This breakdown causes a rapid decrease in the concentration of progesterone and estrogen in the bloodstream. Progesterone maintains the thickened, nutrient-rich uterine lining, called the endometrium, which was built up in preparation for a fertilized egg. When progesterone levels drop significantly, the supportive blood flow and structural integrity of the endometrium cannot be sustained.
This hormonal withdrawal triggers the breakdown of the uterine tissue. The subsequent shedding of this tissue and blood is the menstrual flow, marking the first day of the cycle. During this phase, circulating levels of progesterone are at their lowest (often less than 0.7 ng/mL), and estrogen levels are also near their lowest point. This low hormonal state defines the menstrual phase, which typically lasts three to seven days.
Follicle-Stimulating Hormone Begins Its Ascent
In contrast to the declining steroid hormones, Follicle-Stimulating Hormone (FSH) begins to increase during the menstrual phase. The low concentrations of estrogen and progesterone remove the negative feedback on the pituitary gland. With this inhibition lifted, the pituitary gland secretes more FSH, initiating the next cycle’s preparation.
The initial rise of FSH occurs while the menstrual flow is active, often peaking around day three. This increase serves to recruit new ovarian follicles. Follicles are small, fluid-filled sacs in the ovaries that contain immature eggs.
FSH stimulates a cohort of these follicles to begin growing and maturing in a process called folliculogenesis. This is an important step, as these developing follicles will compete, and eventually, one dominant follicle will be selected to release an egg later in the cycle. FSH concentration during this early follicular phase is typically in the range of 4 to 9.9 mIU/mL.
Luteinizing Hormone Remains Low
While FSH is rising to initiate follicle growth, Luteinizing Hormone (LH) remains at a low, foundational level throughout the menstrual phase. LH is also produced by the pituitary gland, and its secretion is influenced by the low estrogen and progesterone levels. However, its primary role in the reproductive cycle is not yet required.
The level of LH during the menstrual flow is typically between 1 and 12 International Units per liter (IU/L), establishing the baseline for the hormone. LH will not have its major impact until the middle of the cycle, when a rapid surge is needed to trigger the release of the egg.
The low concentration of LH during menstruation indicates the body is in the early stage of cycle preparation. At this time, LH supports the initial growth of the newly recruited follicles alongside FSH. LH levels only begin to climb significantly later in the follicular phase, once growing follicles produce high amounts of estrogen.