A period is the result of a roughly month-long hormonal chain reaction that prepares your body for pregnancy, then resets when pregnancy doesn’t occur. A normal cycle lasts between 24 and 38 days, with 28 days being the most commonly cited average. Understanding how each phase works helps make sense of the bleeding, the cramps, and the shifting symptoms that come with it.
The Four Phases of the Cycle
Your menstrual cycle has four distinct phases that flow into one another: menstruation, the follicular phase, ovulation, and the luteal phase. Each is driven by a different hormonal shift, and together they form a continuous loop from the first day of one period to the first day of the next.
The entire process is controlled by four key hormones. Two of them, FSH and LH, are released by a small gland in your brain and act on your ovaries. The other two, estrogen and progesterone, are produced by the ovaries themselves and act on your uterus. These hormones rise and fall in a specific sequence, and each spike or drop triggers the next event in the cycle.
Menstruation: The Reset
Your period is day one of the cycle. It happens because your body built up a thick, blood-rich lining inside the uterus over the previous weeks, and that lining is no longer needed. The bleeding you see isn’t purely blood. Menstrual fluid contains fragments of uterine tissue, single cells, cellular debris, immune cells, and components of the tissue scaffolding that held the lining together. It sheds in pieces rather than all at once, which is why flow can vary from day to day.
The average person loses about two to three tablespoons of fluid total over the course of a period. That’s less than it often seems. Periods typically last between three and seven days, with heavier flow in the first couple of days tapering off toward the end.
Why Cramps Happen
The uterus is a muscular organ, and to shed its lining, it contracts. Those contractions are driven by chemicals called prostaglandins, which the uterus produces at the start of your period. Higher prostaglandin levels mean stronger contractions and more pain. This is why over-the-counter anti-inflammatory painkillers work well for period cramps: they reduce the amount of prostaglandins your uterus makes, which directly weakens the contractions.
The small blood vessels that fed the lining, called spiral arteries, are disrupted as tissue breaks away. A muscular segment near the surface of the uterine wall acts like a valve, constricting these vessels to control bleeding once each section of tissue has shed.
The Follicular Phase: Building Up Again
Even while you’re still bleeding, the next cycle is already underway. Your brain releases FSH (follicle-stimulating hormone), which signals your ovaries to start growing a new batch of follicles. Each follicle is a tiny fluid-filled sac containing an immature egg. Between days six and 14, several follicles begin to develop, but typically only one will mature fully.
As these follicles grow, they produce rising levels of estrogen. That estrogen does two things at once. In the uterus, it stimulates the lining to regrow and thicken with fresh blood vessels and tissue, preparing a new surface for a potential embryo. In the brain, rising estrogen eventually triggers the next hormonal event.
Ovulation: The 24-Hour Window
Around day 14 in a 28-day cycle, estrogen levels hit a threshold that causes a sudden surge of LH (luteinizing hormone) from the brain. This LH surge is the trigger for ovulation. One mature follicle ruptures and releases its egg into the fallopian tube, where it can be fertilized for roughly 12 to 24 hours.
Some people feel ovulation as a brief twinge or mild cramp on one side of the lower abdomen. Others notice a change in cervical mucus, which becomes clearer and more slippery around this time. Many people feel no physical signs at all.
The Luteal Phase: Waiting for a Signal
After the egg is released, the empty follicle left behind transforms into a temporary structure called the corpus luteum. This small gland has an extraordinary blood supply: every hormone-producing cell within it sits in direct contact with a capillary. Its sole purpose is to pump out progesterone.
Progesterone takes over where estrogen left off, further preparing the uterine lining. The lining becomes spongy and rich with nutrients, creating conditions that would support a fertilized egg implanting and growing. This phase lasts roughly from day 15 to day 28.
If the egg is fertilized and implants, the early embryo sends a hormonal signal that keeps the corpus luteum alive and producing progesterone. If fertilization doesn’t happen, no signal comes. Without that signal, the corpus luteum begins to break down. Its blood vessels deteriorate, immune cells move in, and the structure collapses into a small, nonfunctional scar. Progesterone levels plummet.
How the Drop in Progesterone Starts Your Period
The uterine lining depends on progesterone to survive. When the corpus luteum dies and progesterone falls, the lining loses its hormonal support. The blood vessels feeding it constrict, cutting off oxygen to the tissue. Without blood flow, the top layers of the lining begin to die and detach. Prostaglandin production ramps up, the uterus contracts, and menstruation begins. You’re back to day one.
This is what makes the cycle a true loop. The death of the corpus luteum doesn’t just end one cycle; it removes the hormonal brake that was suppressing FSH. With progesterone gone, the brain is free to release FSH again, and a new set of follicles starts growing before the current period is even over.
How Cycles Change Over a Lifetime
Your cycle won’t always look the same. In the first few years after periods begin, long and irregular cycles are common. The hormonal communication between the brain and ovaries is still calibrating, and ovulation may not happen every month. Over time, cycles tend to shorten and become more predictable.
The most regular years are generally your 20s and 30s, though some variation from month to month is completely normal. A cycle that runs 26 days one month and 30 the next still falls within the healthy range of 24 to 38 days. As you approach menopause, typically in your mid-40s to early 50s, cycles often become irregular again. Periods may come closer together, then farther apart, with heavier or lighter flow, before eventually stopping.
What Menstrual Fluid Actually Contains
Period blood looks different from the blood you’d see from a cut, and that’s because it’s not the same thing. Menstrual fluid is a mix of blood, shed endometrial tissue, single cells, fragments of the structural matrix that held the lining together, immune cells, and various cellular byproducts. The color can range from bright red to dark brown depending on how quickly the fluid moves through the cervix and vagina. Darker blood has simply had more time to oxidize.
Small clots are also normal, particularly on heavier days. These form when the blood pools briefly in the uterus or vagina before being expelled. The tissue shedding is piecemeal, not all at once, which explains why flow intensity shifts throughout a period.