The menstrual cycle is a roughly monthly process in which your body prepares for pregnancy, then resets if pregnancy doesn’t occur. A typical cycle lasts 21 to 35 days, counted from the first day of one period to the first day of the next. It involves a coordinated sequence of hormonal signals between your brain and your ovaries that drives changes in your eggs, your uterine lining, and how you feel day to day.
The Four Phases of the Cycle
The menstrual cycle has four distinct phases, each with its own hormonal profile and physical effects. While a 28-day cycle is often used as a reference point, the follicular phase (the first half) is the most variable in length, which is why total cycle length differs so much from person to person.
Menstrual Phase
The cycle begins with your period. Because no fertilized egg has implanted, levels of estrogen and progesterone drop sharply, and the top layers of your uterine lining shed. Bleeding typically lasts 2 to 7 days. During this time, the lining thins from roughly 16 to 18 millimeters down to just 1 to 4 millimeters. Total blood loss over a normal period is under 60 milliliters, which is about four tablespoons. Loss above 80 milliliters is classified as heavy menstrual bleeding.
Follicular Phase
The follicular phase technically starts on the same day as your period (day 1) and lasts an average of 13 to 14 days. While you’re still bleeding in the early days, your brain’s pituitary gland ramps up production of follicle-stimulating hormone (FSH). This triggers 3 to 30 small follicles in your ovaries to start growing, each containing a single egg. By about day 7, one dominant follicle pulls ahead and begins producing significant amounts of estrogen. The rising estrogen causes the other follicles to die off and signals your uterine lining to start thickening again, reaching about 12 to 13 millimeters before ovulation. Many people notice a boost in energy and better sleep during this phase as estrogen climbs.
Ovulatory Phase
When estrogen reaches a high enough level, usually around day 13, it triggers a sudden surge of luteinizing hormone (LH) from the pituitary gland. This LH surge is the direct trigger for ovulation: the dominant follicle ruptures and releases a mature egg. The entire ovulatory phase is short, lasting only 16 to 32 hours. The egg itself is released about 10 to 12 hours after LH peaks, or roughly 36 to 40 hours after the surge begins. You may notice mild cramping on one side, a slight rise in basal body temperature, and changes in vaginal discharge, which becomes clearer and more slippery around ovulation.
Luteal Phase
After the egg is released, the emptied follicle transforms into a structure called the corpus luteum, which pumps out progesterone along with some estrogen. Progesterone is the dominant hormone of this phase. It makes the uterine lining thicken further, becoming rich with blood vessels and nutrients to support a potential embryo. The luteal phase is the most consistent part of the cycle, lasting about 14 days in most people.
If the egg isn’t fertilized, the corpus luteum breaks down, progesterone and estrogen plummet, and the lining sheds. That drop in hormones is what restarts the cycle. Common luteal phase symptoms include breast tenderness, bloating, food cravings, headaches, constipation, and trouble sleeping. These are the hallmarks of what most people recognize as PMS.
How the Hormones Work Together
Four hormones run the show, and they operate through a feedback loop. FSH kicks things off by growing follicles. Those follicles produce estrogen, which in turn tells the brain to dial FSH back down, ensuring only one follicle matures. When estrogen gets high enough, it flips the switch on LH, creating the surge that triggers ovulation. After ovulation, the corpus luteum produces progesterone, which prepares the uterine lining and suppresses further ovulation for the rest of that cycle.
This feedback system is why hormonal birth control works. Synthetic hormones override the natural signals, preventing the FSH rise that grows follicles or the LH surge that triggers egg release.
What Changes Throughout Your Life
Cycles are often irregular for the first few years after your first period, as the hormonal feedback loop is still maturing. Through your 20s and 30s, cycles tend to become more predictable, though some variation from month to month is completely normal. A cycle that falls anywhere between 21 and 35 days is within the standard range.
In your 40s, perimenopause typically begins. Your ovaries gradually produce less estrogen, and cycles may become longer, shorter, or unpredictable. Perimenopause can last anywhere from a few months to eight or ten years. Irregular periods, hot flashes, and mood changes are common during this transition. Menopause is confirmed after 12 consecutive months without a period.
Signs Your Cycle May Need Attention
Not every irregular cycle signals a problem, but certain patterns are worth paying attention to. The American College of Obstetricians and Gynecologists flags the following as potentially abnormal:
- Cycle length extremes: periods coming more often than every 21 days, less often than every 45 days, or going more than 90 days apart even once.
- Long bleeding: periods lasting more than 7 days.
- Heavy flow: needing to change a pad or tampon more than once every 1 to 2 hours, or feeling dizzy or faint during your period.
- Absent periods: being 15 or older and never having had a period, or previously regular periods that stop being regular without explanation.
Cycle changes can reflect a wide range of things, from stress and weight changes to thyroid issues and polycystic ovary syndrome. Tracking your cycle length, flow, and symptoms for a few months gives you useful data to share if you do seek medical input. Many period-tracking apps make this easy, though a simple calendar works just as well.