A menstrual cycle is a repeating sequence of hormonal shifts that prepares your body for pregnancy each month, then resets when pregnancy doesn’t occur. The full cycle averages 28 days, though anywhere from 21 to 35 days is normal for adults. It involves four overlapping phases, each driven by different hormones that affect your ovaries, uterine lining, and the egg itself.
The Four Phases of the Cycle
Your cycle starts on the first day of your period and ends the day before your next one begins. The four phases don’t happen in strict sequence. Some overlap, and the timing shifts from person to person and even cycle to cycle. Here’s how they break down in a typical 28-day cycle.
Menstruation (Days 1 to 7)
This is the phase you’re most aware of: your period. When no pregnancy has occurred, hormone levels drop, and the thickened lining of your uterus has no reason to stay. Your body releases chemicals called prostaglandins, which trigger the muscular walls of your uterus to contract and squeeze the lining out. Those contractions are what you feel as cramps.
Most people bleed for three to five days, though anything from three to seven days falls within a normal range. Average blood loss is about 30 milliliters per cycle, roughly two tablespoons. That can be hard to gauge in practice, but the amount matters less than the pattern. A sudden, significant change in your flow is more meaningful than comparing yourself to someone else.
The Follicular Phase (Days 1 to 14)
This phase overlaps with menstruation and extends beyond it. It starts when your brain’s pituitary gland releases follicle-stimulating hormone (FSH), which actually begins before your period even starts. FSH does exactly what its name suggests: it stimulates small fluid-filled sacs in your ovaries, called follicles, to start developing. Each follicle contains an immature egg.
As follicles grow, they produce estrogen. By about day seven, estrogen levels in your blood rise significantly. That estrogen signals your uterine lining to start rebuilding. The lining thickens to about 12 to 13 millimeters (roughly half an inch) before ovulation, creating a nutrient-rich environment that could support a fertilized egg. Meanwhile, one dominant follicle pulls ahead of the rest and forms a fully mature egg, typically between days 10 and 14.
Ovulation (Around Day 14)
Rising estrogen from the dominant follicle triggers a sudden surge of luteinizing hormone (LH), usually around day 13. Ovulation, the release of the mature egg from the ovary, happens 28 to 36 hours after this LH surge begins and about 10 to 12 hours after LH peaks. In a 28-day cycle, that puts ovulation at roughly day 14, but it can shift earlier or later depending on your cycle length.
The released egg travels into the fallopian tube, where it can be fertilized by sperm. The egg itself is viable for about 12 to 24 hours. Sperm, however, can survive in the reproductive tract for up to five days. That’s why the fertile window is six days long: the five days before ovulation plus ovulation day itself. The two most likely days for conception are the day before and the day of ovulation.
The Luteal Phase (Days 15 to 28)
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. This structure produces progesterone, the hormone that dominates the second half of your cycle. Progesterone’s job is to further prepare the uterine lining for a potential embryo. During this phase, the lining reaches its maximum thickness of about 16 to 18 millimeters.
If a fertilized egg implants in the lining, the corpus luteum keeps producing progesterone for about 10 weeks, until the placenta takes over. If no implantation occurs, the corpus luteum breaks down, progesterone and estrogen levels drop, and the lining loses its hormonal support. That decline triggers your next period, and the cycle restarts.
How the Hormones Work Together
Four hormones run the show, and they operate on a feedback loop. FSH kicks things off by stimulating follicle growth. Those growing follicles produce estrogen, which rebuilds the uterine lining and eventually triggers the LH surge that causes ovulation. After ovulation, progesterone takes over, maintaining the lining and suppressing further ovulation so the body focuses on one potential pregnancy at a time.
When progesterone drops at the end of the luteal phase, it removes the signal that was keeping the uterine lining intact. It also releases the brake on FSH, allowing follicle development to begin again. This is why the process is called a cycle: the end of one directly triggers the beginning of the next. Your body doesn’t wait for a period to end before starting the follicular phase. The two overlap from day one.
What Your Cycle Looks Like From the Outside
You can’t feel your hormones shifting, but your body gives off signals you can track. Basal body temperature, your temperature first thing in the morning before getting out of bed, rises slightly after ovulation, typically by less than half a degree Fahrenheit (about 0.3°C). When that small increase holds steady for three or more days, ovulation has likely already occurred. This makes temperature tracking more useful for confirming ovulation after the fact than for predicting it in advance.
Cervical mucus also changes throughout the cycle. In the days leading up to ovulation, it becomes clearer, more slippery, and stretchy, often compared to raw egg whites. After ovulation, progesterone makes it thicker and stickier. Paying attention to these changes over a few months can help you recognize your own pattern, which varies from person to person more than textbooks suggest.
What Counts as a Normal Cycle
A “textbook” cycle is 28 days, but that’s just an average. Cycles between 21 and 35 days are considered normal for adults. Adolescents often have longer, more irregular cycles because they may not ovulate every month. It’s statistically uncommon for teens to go more than 90 days between periods, so gaps longer than three months are worth investigating at any age.
Cycle length also varies within the same person. Stress, illness, travel, weight changes, and sleep disruption can all shift the timing of ovulation, which in turn changes how long the cycle lasts. The luteal phase tends to be the more consistent half, usually staying close to 14 days. Most variation happens in the follicular phase, which is why your period might arrive a few days early or late from one month to the next.
Signs That Something May Be Off
Some variations in your cycle are expected. Others point to a hormonal imbalance, a structural issue, or another underlying condition. Bleeding that lasts more than seven days, soaking through a pad or tampon every hour for several consecutive hours, needing to double up on pads, or passing blood clots the size of a quarter or larger all fall outside the normal range for menstrual flow.
Periods that suddenly become much heavier, much lighter, or stop altogether can signal changes in thyroid function, polycystic ovary syndrome, significant weight loss, or other conditions that affect hormone production. Severe pain that doesn’t respond to over-the-counter relief, or pain that gets worse over time, can be a sign of endometriosis or fibroids. Tracking your cycle length, flow, and symptoms for a few months gives you a baseline, which makes it much easier to spot a meaningful change when one happens.