The menstrual phase is the first stage of the menstrual cycle, beginning on day one of your period and lasting three to five days on average. It’s the stretch of time when the uterine lining sheds and exits the body as menstrual fluid. While most people think of it simply as “having your period,” a surprising amount of activity is happening beneath the surface, including the very first steps your body takes toward preparing for the next cycle.
What Happens Inside the Uterus
During the previous cycle, the uterine lining thickened with blood-rich tissue in preparation for a fertilized egg. When no egg implants, levels of both estrogen and progesterone drop sharply. That hormonal withdrawal is the direct trigger for your period. It signals the uterine lining that it’s no longer needed, and the tissue begins to break down.
The drop in progesterone also causes the body to ramp up production of compounds called prostaglandins. These chemicals make the muscular wall of the uterus contract, squeezing the lining away and pushing it out through the cervix. Those contractions are what you feel as menstrual cramps. Higher prostaglandin levels tend to produce stronger, more painful cramps. At the same time, prostaglandins cause the small blood vessels feeding the lining to constrict, temporarily cutting off blood supply to the tissue. The resulting oxygen deprivation helps the lining detach cleanly so it can be shed.
Menstrual fluid is a mix of blood, tissue from the uterine lining, and cervical mucus. Normal blood loss for an entire period falls below about 60 mL, which is roughly four tablespoons. Periods with 60 to 100 mL of blood loss are considered moderately heavy, and anything above 80 to 100 mL crosses into excessive territory. In practical terms, though, most people never measure this directly.
The Hormonal Landscape
The menstrual phase is the hormonal low point of the entire cycle. Estrogen and progesterone are both at their lowest, which is why fatigue, low mood, and low energy are so common during your period. These hormonal dips also affect serotonin, a brain chemical tied to mood regulation. When serotonin drops alongside estrogen, it can contribute to feelings of sadness, food cravings, and difficulty sleeping that carry over from the premenstrual days.
But this hormonal valley doesn’t last long. Even during the first few days of bleeding, the pituitary gland (a small structure at the base of the brain) begins to slightly increase its output of follicle-stimulating hormone, or FSH. That early rise in FSH is the body’s way of kick-starting the next cycle before the current one has even finished.
Your Ovaries Are Already Preparing
Most people assume the ovaries are idle during a period, but they’re not. At the very onset of menstruation, a group of small fluid-filled sacs (about 2 to 5 mm each) is sitting in the ovaries, each containing an immature egg. As FSH levels climb past a critical threshold, these follicles are rescued from dying off and begin to grow. Researchers call this “follicle recruitment,” and it’s the first step in the process that will eventually produce a single mature egg for ovulation later in the cycle.
The timing matters. FSH needs to stay above that threshold long enough for one follicle to pull ahead of the rest and become dominant. This overlap between the menstrual phase and early follicle development is why the menstrual phase is sometimes grouped together with the follicular phase. Technically, the follicular phase encompasses the entire first half of the cycle (from day one of your period through ovulation), and the menstrual phase is simply its opening chapter.
Common Symptoms and Why They Happen
Cramps are the hallmark symptom, driven by prostaglandin-triggered uterine contractions. They’re usually strongest in the first one to two days when prostaglandin levels peak. Some people also feel cramping pain in the lower back or thighs because the nerves serving the uterus overlap with nerves in those areas.
Fatigue during your period has multiple causes. The hormonal low point plays a role, but so does the physical process of losing blood, especially for people with heavier flows. Bloating, headaches, and breast tenderness that started in the premenstrual days often linger into the first day or two of bleeding before gradually easing as the period progresses. Loose stools or mild digestive upset are also common because prostaglandins can stimulate smooth muscle throughout the body, not just in the uterus. When they reach the intestines, they speed things up.
Normal vs. Abnormal Bleeding
A period lasting three to seven days falls within the normal range. Cycles that arrive fewer than 21 days apart or more than 35 days apart are considered irregular and worth investigating. The amount of bleeding matters too. Soaking through a pad or tampon every hour for two to three consecutive hours, or passing blood clots larger than a quarter, are signs of unusually heavy bleeding.
Heavy periods aren’t just inconvenient. Over time, they can lead to iron deficiency, which compounds the fatigue and brain fog that many people already experience during menstruation. If your period consistently disrupts your ability to go about your day, or if you notice a significant change in flow, duration, or clot size compared to what’s typical for you, that shift is worth bringing up with a healthcare provider.
How the Menstrual Phase Fits Into the Full Cycle
The average menstrual cycle runs about 28 days, though anywhere from 21 to 35 days is normal. The menstrual phase occupies roughly the first three to seven of those days. After bleeding stops, the rest of the follicular phase continues as estrogen rises and the uterine lining begins rebuilding. Ovulation typically occurs around the midpoint of the cycle, followed by the luteal phase, when progesterone takes over to maintain the lining. If pregnancy doesn’t occur, progesterone drops again, and the whole sequence resets with a new period.
Understanding where the menstrual phase sits in this loop helps explain why symptoms shift throughout the month. The low-hormone environment of your period is the mirror image of the high-progesterone luteal phase that precedes it. Each phase creates a distinct hormonal climate, and the transition between them is what drives both the physical and emotional changes you feel across the cycle.