A menstrual cycle is the monthly process your body goes through to prepare for a possible pregnancy. It typically lasts between 21 and 35 days, with 28 days being the average. The cycle has four distinct phases, each driven by shifting hormone levels that affect everything from your mood to your energy to the consistency of your cervical mucus.
The Four Phases at a Glance
Your cycle is counted starting from the first day of your period. From there, it moves through four phases: menstruation, the follicular phase, ovulation, and the luteal phase. Each one is triggered by rising or falling levels of four key hormones produced by your brain and ovaries. These phases overlap slightly, but together they form a predictable rhythm that repeats until menopause.
Menstruation (Days 1 to 7)
Your period marks the beginning of a new cycle. It happens because progesterone levels drop sharply when no fertilized egg has implanted in the uterine lining. That drop sets off a chain reaction: blood vessels in the lining become fragile and permeable, immune cells flood the tissue, and enzymes begin breaking down the lining’s structural framework. The result is the shedding of blood and tissue through the vagina.
Most periods last between 2 and 7 days. Flow is usually heaviest in the first day or two, then tapers off. You may also feel cramping in your lower abdomen during this phase, caused by the uterus contracting to help shed its lining. About one in four women experiences unusually heavy bleeding, which can signal an underlying issue worth investigating.
The Follicular Phase (Days 1 to 13)
The follicular phase actually overlaps with menstruation, starting on day 1 and lasting until ovulation. During this time, your brain’s pituitary gland releases a hormone that stimulates your ovaries to begin developing several small fluid-filled sacs called follicles. Each follicle contains an immature egg.
As the phase progresses, one follicle becomes dominant. It outcompetes the others and begins producing rising amounts of estrogen. That estrogen does two critical things: it signals the uterus to start rebuilding its lining (thicker, blood-rich, ready for a potential embryo), and it triggers the hormonal surge that launches ovulation. Many people notice increased energy, improved mood, and changes in cervical mucus during this phase as estrogen climbs.
Ovulation (Around Day 14)
Ovulation is the shortest phase, lasting roughly 24 hours. It’s triggered when rising estrogen from the dominant follicle causes a sharp spike in luteinizing hormone (LH). About 36 to 40 hours after that LH surge begins in the bloodstream, the dominant follicle ruptures and releases a mature egg into the fallopian tube.
Home ovulation test kits work by detecting LH in urine. Once the test turns positive, ovulation typically occurs within 12 to 24 hours. The released egg is viable for a relatively short window, generally 12 to 24 hours. This is why the fertile window is narrow: it includes the few days before ovulation (since sperm can survive several days inside the reproductive tract) plus the day of ovulation itself.
The Luteal Phase (Days 15 to 28)
After the egg is released, the ruptured follicle doesn’t just disappear. Its cells transform into a small, yellowish structure called the corpus luteum. This temporary gland pumps out progesterone and estrogen, which keep the uterine lining thick and prepared for implantation.
If sperm does not fertilize the egg, the corpus luteum begins to break down about 10 days after ovulation. As it degrades, progesterone and estrogen levels plummet. Without those hormones sustaining it, the uterine lining destabilizes and sheds, starting your period and beginning the cycle all over again. The luteal phase is fairly consistent across women, lasting close to 14 days regardless of total cycle length. This means that variation in cycle length is mostly due to differences in how long the follicular phase takes.
The second half of the luteal phase is when many people experience premenstrual symptoms: bloating, breast tenderness, irritability, fatigue, or food cravings. These are driven by the rapid hormonal decline as the corpus luteum winds down.
What Counts as a Normal Cycle
A cycle anywhere from 21 to 35 days is considered normal. Period bleeding lasting 2 to 7 days also falls within the typical range. Your cycle length can vary by a few days from month to month without being a concern.
Cycles that fall outside these ranges have specific clinical names. Periods arriving fewer than 21 days apart are considered too frequent. Periods spaced more than 35 days apart are considered infrequent. Not getting a period for 3 or more months (without pregnancy) is classified as absent menstruation. Bleeding that consistently lasts longer than 8 days, or periods that are routinely shorter than 2 days, also fall outside normal patterns. A variation of more than 20 days between cycles is another sign of irregularity.
How Your Cycle Changes Over Time
Your cycle won’t look the same at 15 as it does at 45. In the first few years after menstruation begins, cycles are often irregular as the hormonal system matures. By the late teens and twenties, most people settle into a more predictable pattern.
Starting in the late 30s or 40s, cycles begin shifting again during a transition called perimenopause. Estrogen levels become more erratic, rising and falling unpredictably rather than following a smooth pattern. You may notice cycles getting shorter or longer, flow becoming heavier or lighter, or skipped periods altogether as ovulation becomes less reliable. If your cycle length starts varying by 7 or more days from its usual pattern, that’s often an early sign of this transition. Going 60 or more days between periods suggests you’re further along. Some women notice these changes as early as their 30s, though most experience them in their 40s or early 50s.
Spotting between periods, at any age, is worth paying attention to. While it can be harmless, intermenstrual bleeding is one of the irregularities that may point to hormonal imbalances, structural issues, or other conditions that benefit from evaluation.