The menstrual cycle is the monthly process your body goes through to prepare for a possible pregnancy. It involves a coordinated sequence of hormonal changes that trigger an egg to develop and release from the ovaries, thicken the lining of the uterus, and then shed that lining as a period if pregnancy doesn’t occur. The average cycle lasts 28 days, but anything from 21 to 35 days is normal for adults.
Most people get their first period around age 12, though it can start earlier or later. Periods continue until menopause, which happens at an average age of 52 in the United States. That means the menstrual cycle is active for roughly four decades of life.
The Four Phases of the Cycle
The menstrual cycle has four distinct phases, each driven by shifting levels of hormones. These phases overlap between what’s happening in the ovaries (where the egg develops) and what’s happening in the uterus (where the lining builds up or sheds). Here’s how they unfold.
Menstruation (Days 1 to 7)
Day one of your cycle is the first day of your period. This is when the uterus sheds its inner lining because no pregnancy occurred in the previous cycle. Most people bleed for three to seven days, losing less than 60 milliliters of blood total, which is roughly four tablespoons. Blood loss above 80 milliliters per cycle is considered excessive and worth discussing with a provider.
The Follicular Phase (Days 1 to 13)
This phase overlaps with menstruation and continues after bleeding stops. The pituitary gland, a small structure at the base of the brain, releases a hormone called FSH (follicle-stimulating hormone). FSH tells the ovaries to start developing several small fluid-filled sacs called follicles, each containing an immature egg. Usually one follicle becomes dominant and continues growing while the others dissolve.
As the dominant follicle grows, it produces rising levels of estrogen. That estrogen does two important things: it signals the uterine lining to start thickening and rebuilding after menstruation, and it eventually triggers the next phase. During this rebuilding period, blood vessels and glands in the uterine lining multiply rapidly, creating a nutrient-rich environment.
Ovulation (Around Day 14)
When estrogen reaches a high enough level, the pituitary gland responds with a sharp spike in luteinizing hormone, known as the LH surge. This surge triggers ovulation about 36 to 40 hours later. The mature follicle, now between 1.8 and 2.5 centimeters across, ruptures and releases the egg into the fallopian tube. The ovulatory phase itself is short, lasting only 16 to 32 hours.
This is the fertile window. The egg survives for about 12 to 24 hours after release, and sperm can live in the reproductive tract for up to five days, so the window for conception spans roughly six days around ovulation.
The Luteal Phase (Days 15 to 28)
After releasing the egg, the ruptured follicle closes up and transforms into a structure called the corpus luteum. Its primary job is producing progesterone, the hormone that dominates the second half of the cycle. Progesterone further thickens the uterine lining, filling it with fluids and nutrients that could support an embryo. It also causes the mucus in the cervix to thicken, creating a barrier against bacteria, and raises your basal body temperature slightly.
Both estrogen and progesterone are high during this phase, which is why many people notice breast swelling and tenderness in the two weeks before their period. The rising hormones cause milk ducts in the breasts to widen.
If the egg isn’t fertilized, the corpus luteum breaks down about 10 days after ovulation. Without the progesterone it was producing, the uterine lining can no longer sustain itself. It begins to shed, your period starts, and the cycle resets to day one.
What a Normal Cycle Looks Like
There’s a wide range of normal. Adults typically have cycles between 21 and 35 days, with bleeding lasting seven days or fewer. Teenagers have even more variation: 90% of adolescent cycles fall between 21 and 45 days, and it can take up to three years after a first period for cycles to settle into a regular pattern. By the third year after the first period, 60 to 80% of cycles fall within the typical adult range of 21 to 34 days.
Some signs suggest something may be off. A cycle that consistently comes more often than every 21 days or less often than every 45 days is worth paying attention to. The same goes for periods that last longer than seven days, cycles that are more than 90 days apart (even once), or periods that were regular and then suddenly became unpredictable. Not having a period at all by age 15 is also a reason to check in with a healthcare provider.
Common Symptoms Throughout the Cycle
Because your hormone levels are constantly shifting, you may notice predictable patterns in how you feel at different points in the cycle. During menstruation, cramps are common as the uterus contracts to shed its lining. Some people also experience headaches, fatigue, or lower back pain.
In the days leading up to your period (the late luteal phase), dropping progesterone and estrogen levels can trigger what’s broadly called PMS: mood swings, food cravings, bloating, acne, breast tenderness, and irritability. Cyclic migraines, which appear at the same point in the cycle each month, are also tied to these hormonal shifts. These symptoms vary enormously from person to person, and even from cycle to cycle.
Why Tracking Your Cycle Matters
Clinicians increasingly treat the menstrual cycle as a vital sign, on par with blood pressure or heart rate. Changes in your cycle can be early indicators of conditions that might not show up on standard tests for years. Tracking helps you spot those changes before they become obvious problems.
You don’t need anything complicated. Recording the start date of each period, how many days you bleed, and any symptoms you notice (cramps, mood changes, headaches, spotting between periods) gives you a baseline. Over several months, patterns emerge. You might notice that your migraines always hit two days before your period, or that your cycles have gradually gotten longer. That information becomes incredibly useful in any conversation with a healthcare provider, because memory alone tends to blur those details.
Tracking is also practical for anyone trying to conceive or avoid pregnancy. Ovulation predictor kits measure the LH surge in urine and can tell you ovulation is likely within 12 to 24 hours. Basal body temperature charting can confirm that ovulation happened, though it can’t predict it in advance since the temperature rise occurs after the egg has already been released.
How the Cycle Changes Over a Lifetime
The menstrual cycle isn’t static. In the first few years after menarche (the first period), cycles tend to be longer, heavier, and less predictable as the hormonal communication between the brain and ovaries matures. Through the twenties and thirties, cycles generally become more regular and predictable.
Starting in the mid-forties for most people, the transition toward menopause begins. This phase, called perimenopause, can last eight to ten years. During this time, the ovaries gradually produce less estrogen, and cycles may become shorter, longer, heavier, lighter, or skip months entirely. Menopause itself is defined as going 12 consecutive months without a period, and in the United States, that happens at an average age of 52.