The menstrual cycle is a roughly monthly process in which the body prepares for pregnancy, then resets if pregnancy doesn’t happen. A normal cycle ranges from 21 to 35 days, with the average falling around 28 days. The entire process is driven by four hormones working in sequence, and it plays out simultaneously in two places: the ovaries (where eggs develop and release) and the uterus (where the lining builds up and sheds).
The Four Phases at a Glance
The cycle is counted starting from the first day of your period. In a typical 28-day cycle, it breaks down like this:
- Menstruation (days 1 to 5): The uterine lining sheds. Most people bleed for three to five days, though anywhere from three to seven days is considered normal.
- Follicular phase (days 1 to 13): Overlaps with menstruation and continues after bleeding stops. Eggs begin maturing inside the ovaries.
- Ovulation (around day 14): A mature egg releases from one ovary into the fallopian tube.
- Luteal phase (days 15 to 28): The body prepares the uterine lining for a possible pregnancy. If no pregnancy occurs, the cycle resets.
These phases overlap and flow into each other rather than switching cleanly. The follicular and luteal phases describe what’s happening in the ovaries, while menstruation and the buildup of the uterine lining describe what’s happening in the uterus. Both tracks run in parallel, coordinated by hormones.
How Hormones Drive Each Phase
Four hormones orchestrate the cycle. Two come from the brain’s pituitary gland, and two come from the ovaries themselves.
At the start of the cycle, the pituitary gland releases follicle-stimulating hormone (FSH), which does exactly what its name suggests: it stimulates several follicles in the ovaries to start growing. Each follicle contains an immature egg. As these follicles grow, they produce estrogen, which rises steadily through the first half of the cycle. Estrogen does double duty. In the ovaries, it helps the follicles mature. In the uterus, it causes the lining to thicken and grow new blood vessels, rebuilding what was shed during the previous period.
By mid-cycle, estrogen levels peak. That spike triggers a sudden surge of luteinizing hormone (LH) from the pituitary, typically around day 14. This LH surge is the ovulation trigger. Within 24 to 48 hours, the dominant follicle ruptures and releases its egg. Ovulation itself happens roughly 8 to 20 hours after LH reaches its highest point.
After ovulation, the now-empty follicle transforms into a temporary structure called the corpus luteum. This structure pumps out progesterone, the hormone that dominates the second half of the cycle. Progesterone increases blood flow to the uterus, promotes nutrient-rich secretions in the lining, and reduces contractions of the uterine wall. All of this makes the uterus more receptive to a fertilized egg.
If fertilization doesn’t happen, the corpus luteum starts to break down about 10 days after ovulation. As it deteriorates, progesterone levels plummet. Without progesterone to maintain it, the thickened uterine lining can no longer sustain itself. It begins to break apart, and menstruation starts, resetting the cycle to day one.
What Happens in the Uterus
While the ovaries cycle through follicle growth and ovulation, the uterus goes through its own parallel transformation. After menstruation clears out the old lining, rising estrogen drives a rebuilding phase. The lining grows thicker and develops a rich network of blood vessels. Estrogen also changes the mucus at the cervix (the opening of the uterus), making it thinner so sperm can pass through more easily.
After ovulation, progesterone takes over and shifts the lining into a secretory state. Rather than continuing to grow, the lining now becomes spongy and nutrient-rich, essentially preparing a landing pad for a fertilized egg. If an embryo implants, it sends a hormonal signal that keeps the corpus luteum alive, and the lining stays intact. If no implantation happens, the drop in progesterone causes the blood vessels in the lining to constrict and the tissue to break down. That tissue, mixed with blood, is what you see during your period. The total amount lost is typically about 2 to 3 tablespoons over the course of 4 to 5 days.
Signs Your Body Gives You
The hormonal shifts throughout the cycle produce physical changes you can actually observe. One of the most reliable is cervical mucus. In the days after your period, discharge tends to be dry or sticky, white or slightly yellow. As estrogen rises through the middle of the cycle, it becomes creamy and cloudy, then transitions to a wet, slippery, stretchy consistency that resembles raw egg whites. This egg-white mucus typically lasts about three to four days around ovulation and exists for a biological reason: its slippery texture helps sperm travel more efficiently. After ovulation, progesterone dries things up again, and discharge stays minimal until your next period.
Body temperature also shifts. After ovulation, the progesterone produced by the corpus luteum raises your resting (basal) body temperature by a small but measurable amount, typically between 0.4°F and 1°F. The temperature stays elevated through the luteal phase and drops back down when your period starts. This shift only confirms ovulation after it has already happened, so it’s more useful for tracking patterns over several months than for predicting ovulation in real time.
What Counts as a Normal Cycle
The 28-day cycle is an average, not a rule. For most adults, anything from 21 to 35 days is normal, and for teenagers, the range extends up to 45 days as hormonal patterns are still establishing themselves. Your cycle length can also vary from month to month. A variation of a few days between cycles is typical and not a sign of a problem.
Cycles that consistently fall outside the 21-to-35-day window, or that vary by more than nine days from one cycle to the next, are considered irregular. Missing three or more periods in a row, or going 90 days or more without a period (when you’re not pregnant, breastfeeding, or in menopause), is also clinically abnormal and worth investigating.
When Bleeding Is Heavier Than Normal
Since the cycle resets through shedding the uterine lining, the amount of bleeding matters. Normal flow lasts about 4 to 5 days and totals around 2 to 3 tablespoons of blood. Signs that bleeding is heavier than typical include soaking through a pad or tampon every hour for several consecutive hours, needing to double up on pads, having to change protection overnight, bleeding for more than seven days, or passing clots the size of a quarter or larger. Heavy periods can lead to iron deficiency over time, so persistent heavy flow is worth bringing up with a healthcare provider.