The menstrual cycle is a repeating sequence of hormonal shifts that prepares your body for pregnancy each month. A typical cycle lasts 21 to 35 days in adults, counted from the first day of one period to the first day of the next. The average blood loss during a period is about 30 mL, roughly two tablespoons, and normal flow lasts seven days or fewer. The whole process is driven by a conversation between your brain and your ovaries, with four key hormones orchestrating changes in your ovaries and uterus simultaneously.
The Hormones Running the Show
Your menstrual cycle is controlled by a chain of signals that starts in a small region at the base of your brain called the hypothalamus. It releases a signaling hormone in rhythmic pulses, which tells the pituitary gland (a pea-sized gland just below it) to produce two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These two hormones travel through your bloodstream to your ovaries, where they trigger the production of estrogen and progesterone. Those ovarian hormones then loop back to the brain, either encouraging or suppressing further signals depending on where you are in your cycle.
This feedback loop is what makes the cycle self-regulating. Rising estrogen from a maturing egg follicle gradually sensitizes the pituitary so it becomes more responsive. Just before ovulation, that sensitivity reaches a tipping point and the pituitary releases a large burst of LH, which is the direct trigger for the egg’s release. After ovulation, progesterone takes over as the dominant hormone and suppresses those brain signals, preventing a second egg from being released in the same cycle.
Phase 1: Follicular Phase
The follicular phase begins on day one of your period and lasts until ovulation. In a 28-day cycle, that’s roughly days 1 through 14, though this phase is the most variable part of the cycle and can be shorter or longer from person to person.
During this phase, FSH stimulates several small follicles in your ovaries to start growing. Each follicle contains an immature egg. By around days 10 to 14, one follicle has outpaced the others and becomes the dominant follicle, while the rest stop developing. This winning follicle produces increasing amounts of estrogen, which does two important things: it signals the uterine lining to start thickening, and it primes the pituitary gland for the LH surge that will trigger ovulation.
Meanwhile, your uterine lining is rebuilding after menstruation. It starts at just 3 to 4 mm thick after your period ends, then grows to about 7 to 8 mm by the end of this phase as estrogen stimulates new blood vessels, glands, and tissue layers to form.
Phase 2: Ovulation
Ovulation is the shortest phase, lasting roughly 24 to 36 hours. It happens around day 14 in a 28-day cycle, triggered by that sudden surge of LH from the pituitary gland. The dominant follicle ruptures and releases its mature egg into the fallopian tube, where it can survive for about 12 to 24 hours waiting for fertilization.
This is the point in your cycle when your body gives the clearest physical signals. Your basal body temperature rises slightly after ovulation, typically less than half a degree Fahrenheit (0.3°C). Cervical mucus becomes slippery, stretchy, and clear, often compared to raw egg whites. This texture helps sperm travel more easily. You may notice this fertile-quality mucus for about three to four days around ovulation, roughly days 10 to 14 of a 28-day cycle.
Phase 3: Luteal Phase
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. This structure produces rising amounts of progesterone, along with estrogen, during the roughly 14 days between ovulation and your next period (days 15 to 28).
Progesterone is the hormone that prepares the uterine lining for a potential pregnancy. It shifts the lining from simply growing thicker to actively secreting nutrients and developing the blood supply a fertilized egg would need. By the end of the luteal phase, the lining reaches 8 to 12 mm thick, and in the two days before menstruation it can reach 12 to 16 mm. Progesterone also converts the more potent form of estrogen in the uterus into a weaker form, keeping the lining’s growth controlled.
During this phase, cervical mucus dries up and becomes thick or sticky again, and your basal body temperature stays slightly elevated compared to the first half of your cycle.
What Happens if There’s No Pregnancy
If the egg isn’t fertilized, the corpus luteum has a built-in expiration date. Around 10 to 12 days after ovulation, it begins to break down. This degradation happens in two stages: first the corpus luteum stops producing progesterone (functional regression), and then its structure physically shrinks (structural regression). A chemical signal called prostaglandin F2-alpha, produced primarily by the uterus, plays a key role in shutting down progesterone production.
The drop in both progesterone and estrogen is what triggers menstruation. Without hormonal support, the thickened uterine lining can no longer sustain itself. Blood vessels in the lining constrict and then break down, and the tissue begins to shed. This falling hormone level also releases the brain from suppression, allowing FSH to rise again and start recruiting a new batch of follicles. The next cycle has already begun.
What Menstrual Fluid Actually Contains
Menstrual fluid is not just blood. It’s a mix of blood, mucus, and shed endometrial tissue. That tissue includes the glands, connective tissue cells, and fragments of the spiral arteries that had developed to nourish a potential pregnancy. This is why menstrual fluid often looks different from blood you’d see from a cut: it can be darker, thicker, and contain small clots or tissue pieces. Using three to six pads or tampons per day is considered typical flow. Needing to change products every one to two hours, especially over more than seven days, is considered excessive and can eventually lead to anemia.
How to Track Your Cycle
Tracking your cycle gives you practical information about whether you’re ovulating regularly and when your fertile window falls. The simplest method is recording when your period starts each month and noting the length of each cycle. Most adults settle into cycles between 21 and 34 days, though teens commonly have longer cycles (up to 45 days) in the first few years after their first period.
Cervical mucus is one of the most accessible daily indicators of where you are in your cycle. After your period, discharge is typically dry or sticky and white or yellowish. It gradually becomes creamier and wetter, then transitions to the clear, stretchy, egg-white consistency that signals peak fertility. After ovulation, it returns to dry or sticky. If you never notice that egg-white mucus, it could be a sign you’re not ovulating that cycle.
Basal body temperature is another tracking tool, though it confirms ovulation after the fact rather than predicting it. You take your temperature first thing in the morning before getting out of bed. After ovulation, you should see a small but consistent temperature shift that stays elevated until your next period. If your temperature stays flat throughout the cycle, that’s another indicator ovulation may not have occurred.
When Ovulation Doesn’t Happen
Not every cycle includes ovulation. An anovulatory cycle occurs when your ovaries don’t release an egg, usually because of a hormonal imbalance. You can still have bleeding during an anovulatory cycle, which makes it easy to assume everything is normal. But the bleeding pattern is often different: periods may be unusually heavy (more than 80 mL of blood loss), unusually light (less than 20 mL), or irregular in timing.
Occasional anovulatory cycles are common during the first few years of menstruation and during the transition to menopause. They can also be triggered by stress, significant weight changes, or conditions affecting hormone balance. The clearest signs that ovulation didn’t happen are the absence of egg-white cervical mucus around mid-cycle, no temperature shift on a basal body temperature chart, and irregular cycle lengths. If anovulation becomes a pattern, it affects both fertility and the health of the uterine lining, since without progesterone from the corpus luteum, the lining may grow unevenly under estrogen alone.