A period cycle, or menstrual cycle, is the monthly process your body goes through to prepare for a possible pregnancy. It starts on the first day of your period and ends the day before your next period begins. The average cycle lasts 28 days, but anything between 21 and 35 days is normal. Each cycle moves through four distinct phases driven by shifting hormones, and understanding those phases can tell you a lot about your overall health.
How to Count Your Cycle
Day 1 is always the first day of menstrual bleeding, not the last. You count forward from there until the day before your next period starts. That total number of days is your cycle length. Most people bleed for two to seven days, and the amount of blood lost varies widely. Direct measurements using menstrual cups put the average volume at about 87 milliliters across a full period, though anything from 15 to around 160 milliliters falls within the normal range.
Tracking your cycle for a few months gives you a baseline. If your cycles consistently fall between 21 and 35 days and your bleeding lasts seven days or fewer, your cycle is considered regular. In the first year or two after periods start (the median age is about 12), cycles tend to be longer and less predictable, with an average interval of around 32 days. That’s normal for adolescents, whose cycles can range from 21 to 45 days as the hormonal system matures.
Phase 1: Menstruation
Menstruation is the shedding of the uterine lining that built up during the previous cycle. When no pregnancy occurs, hormone levels drop, and the lining breaks down and exits through the vagina as menstrual blood. This phase typically lasts three to seven days. Using three to six pads or tampons per day is a common benchmark for normal flow.
Phase 2: The Follicular Phase
The follicular phase overlaps with menstruation and extends beyond it. It covers roughly the first half of your cycle. During this time, your pituitary gland (a small gland at the base of the brain) releases a hormone that signals your ovaries to start developing follicles. Each follicle is a tiny fluid-filled sac containing an immature egg. Several follicles begin developing, but one eventually becomes dominant.
That dominant follicle pumps out rising levels of estrogen, which does two important things. First, it thickens the uterine lining so it could support a fertilized egg. Second, it signals the pituitary gland to slow down follicle production, causing the remaining follicles to wither away and get reabsorbed by the body. You may also notice changes in cervical mucus during this phase: it starts out thick, white, and dry, then gradually becomes creamier as estrogen rises.
Phase 3: Ovulation
Ovulation is the release of a mature egg from the ovary. It happens roughly midcycle, typically a couple of weeks after your period starts. The trigger is a rapid surge in luteinizing hormone (LH) from the pituitary gland. That surge causes the dominant follicle to rupture and release the egg into the fallopian tube, where it can be fertilized for about 12 to 24 hours.
Your body gives off a few signals around ovulation. Cervical mucus becomes clear, slippery, and stretchy, often compared to raw egg whites. This texture helps sperm travel more easily. Some people also notice a slight twinge of pain on one side of the lower abdomen, mild breast tenderness, or a subtle increase in basal body temperature. These signs aren’t universal, but they’re common enough that many people use them to estimate their fertile window.
Phase 4: The Luteal Phase
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. Its job is to produce progesterone, the hormone that maintains and enriches the uterine lining, making it a supportive environment for a potential embryo to implant and grow.
If the egg isn’t fertilized, the corpus luteum breaks down after about 10 to 16 days. Progesterone and estrogen levels fall sharply, the thickened lining can no longer sustain itself, and menstruation begins. This hormonal drop is also what triggers many premenstrual symptoms: bloating, mood changes, breast tenderness, and fatigue. During this phase, cervical mucus returns to being thick and dry.
If the egg is fertilized and implants, the corpus luteum keeps producing progesterone to support early pregnancy until the placenta takes over. This is why a missed period is often the first sign of pregnancy.
What Your Cycle Says About Your Health
The American College of Obstetricians and Gynecologists considers the menstrual cycle an additional vital sign, on par with blood pressure and heart rate. Abnormal cycle patterns can serve as early warning signs for a range of health conditions, which is why clinicians are encouraged to ask about the first day of your last period at every routine visit.
Keeping track of your cycle length, flow, and symptoms gives you a personal baseline. Significant changes from that baseline, such as cycles suddenly becoming much shorter, longer, or heavier, can signal hormonal shifts worth investigating.
Common Causes of Irregular Cycles
Irregular periods have many possible causes. Some of the most common include:
- Polycystic ovary syndrome (PCOS) and other hormonal imbalances
- Thyroid dysfunction, whether the thyroid is overactive or underactive
- Chronic stress and excessive exercise, both of which can suppress the hormones that drive ovulation
- Eating disorders such as anorexia or bulimia, which disrupt hormone production
- Perimenopause, which can begin eight to ten years before menopause, usually in your 40s, as estrogen production gradually declines
- Hormonal birth control, including pills, implants, injections, and hormone-containing IUDs
- Uncontrolled diabetes and elevated cortisol levels
- Certain medications, including some used for epilepsy and mental health conditions
Pregnancy is also a common reason for a missed or late period and is usually the first thing to rule out.
Signs Your Cycle May Need Attention
Not every variation is a problem, but certain patterns are worth paying attention to. Cycles consistently shorter than 21 days or longer than 35 days fall outside the typical range. Bleeding that lasts more than seven days, soaking through a pad or tampon every hour for several hours, or passing large clots can indicate heavy menstrual bleeding. Periods that suddenly stop for three or more months (when you’re not pregnant) or cycles that were once regular and become unpredictable also warrant a closer look. The corpus luteum sometimes doesn’t produce enough progesterone, which can make it difficult to get pregnant or maintain an early pregnancy.
Your cycle is a recurring feedback loop between your brain, ovaries, and uterus. When one part of that system shifts, the cycle reflects it. That’s exactly why tracking it is so useful: it gives you a window into what’s happening hormonally long before other symptoms show up.