A period happens when the lining of your uterus sheds because your body didn’t become pregnant that cycle. The whole process is driven by rising and falling hormone levels that follow a roughly monthly pattern, typically repeating every 21 to 35 days. Understanding what’s happening at each stage makes it easier to recognize what’s normal for your body and what isn’t.
The Four Phases of Your Cycle
Your menstrual cycle isn’t just “period” and “not period.” It moves through four distinct phases, each controlled by different hormones. Day one is the first day of bleeding.
Menstruation (days 1 through 3–7): The uterine lining breaks down and leaves your body as menstrual flow. Bleeding typically lasts two to seven days.
Follicular phase (days 1 through ~13): This phase overlaps with menstruation and continues after bleeding stops. Your brain sends a hormone signal that tells your ovaries to start maturing several egg-containing follicles. As these follicles grow, they release estrogen, which thickens the uterine lining again in preparation for a potential pregnancy.
Ovulation (~day 14): A surge of another brain hormone triggers the release of one mature egg from an ovary. The egg travels into the fallopian tube, where it can be fertilized for about 12 to 24 hours.
Luteal phase (days ~15 through 28): After the egg is released, the empty follicle transforms into a temporary structure that pumps out progesterone. This hormone stabilizes and further enriches the uterine lining, making it spongy and blood-vessel-rich, essentially a cushion ready to support a fertilized egg. The luteal phase is the most consistent part of the cycle, averaging 12 to 14 days with a normal range of 10 to 17 days. When cycles vary in total length, it’s almost always because the follicular phase was shorter or longer, not because the luteal phase changed.
What Actually Triggers Bleeding
If no fertilized egg implants in the uterine lining, the structure producing progesterone breaks down. Progesterone levels drop sharply, and that withdrawal is the direct trigger for menstruation. The lining can only stay intact when progesterone holds it in place. Once progesterone falls, the blood vessels in the lining constrict, cutting off the oxygen supply to the upper layers of tissue. Those layers die, detach, and leave the body along with blood from the exposed blood vessels underneath.
This is why researchers describe the menstruating uterus as a “wounded” surface that has to rapidly repair itself every month. Within hours of shedding, the lower layer of the lining is already regenerating, and by the time bleeding stops, a fresh lining is growing under the influence of rising estrogen from the next batch of developing follicles. The entire repair process happens without scarring, which is unusual for the human body.
Why Periods Cause Cramps
The uterus doesn’t passively let the lining fall away. It actively contracts to push the tissue out, and those contractions are what you feel as cramps. The contractions are caused by chemical messengers that the lining itself produces as it breaks down. Higher levels of these messengers mean stronger contractions and more pain.
This is also why common pain relievers like ibuprofen and naproxen work well for period cramps. They block the production of these chemical messengers, reducing both the intensity of contractions and the inflammation around them. They tend to work best when taken at the first sign of cramping or even just before your period starts, because they prevent the messengers from building up rather than trying to counteract them after the fact. People who produce more of these messengers than average tend to have noticeably more painful periods, and this is one of the most common reasons people seek medical care related to menstruation.
How Much Blood Is Normal
Periods feel heavier than they actually are. The most common total blood loss over an entire period is about two tablespoons (30 ml), and the normal range runs from 10 to 35 ml. Menstrual flow also contains tissue and mucus, so the volume on your pad or tampon isn’t pure blood.
Heavy menstrual bleeding is medically defined as losing more than 80 ml per period, which works out to roughly 16 fully soaked regular-sized pads or tampons across the whole period. In practical terms, a more useful threshold is 9 to 12 soaked products per period. Signs that your flow may be heavier than normal include soaking through a pad or tampon every hour for several consecutive hours, needing to double up on products, or having to change protection during the night. Passing blood clots larger than a quarter is another signal worth paying attention to.
What Varies and What Stays Consistent
A “textbook” 28-day cycle is just an average. Cycles anywhere from 21 to 35 days are considered normal, and it’s common for your own cycle length to shift by a few days from month to month. Stress, sleep changes, significant weight shifts, and intense exercise can all lengthen or shorten a cycle, primarily by delaying or accelerating ovulation.
Bleeding duration is similarly variable. Two days of flow and seven days of flow are both within the normal range. Your flow pattern often follows a predictable shape for you personally: many people experience heavier flow on days one and two, tapering to lighter flow or spotting by the end. Others have a slow start, a heavier middle, and a trailing finish. What matters more than matching a textbook pattern is knowing your own baseline so you can spot meaningful changes.
Cycles shorter than 21 days, longer than 35 days, or bleeding that stretches beyond seven days consistently are worth investigating. The same goes for cycles that were previously regular and become unpredictable, since that shift can reflect changes in ovulation, thyroid function, or other hormonal patterns.
How Hormonal Birth Control Changes the Process
Hormonal contraceptives work by overriding the natural hormone cycle. Most methods supply steady levels of synthetic versions of estrogen, progesterone, or both, which prevent the brain from sending the signals that trigger follicle development and ovulation. Without ovulation, there’s no progesterone surge and drop, and the uterine lining stays thin.
The “period” you get during the placebo week of a pill pack isn’t a true period. It’s withdrawal bleeding caused by the sudden drop in synthetic hormones, mimicking the natural progesterone withdrawal that triggers real menstruation. Because the lining is thinner on hormonal birth control, this bleeding is typically lighter and less painful than a natural period. Some methods skip the placebo week entirely, which is why certain contraceptives reduce or eliminate bleeding altogether.