The menstrual cycle is the monthly hormonal process your body goes through to prepare for pregnancy. It’s counted from the first day of one period to the first day of the next, and a normal cycle lasts between 24 and 38 days. If your cycle consistently falls within that range, it’s considered regular, even if it doesn’t land on exactly 28 days every time.
The cycle involves a coordinated sequence of hormone shifts that affect the ovaries, uterus, and the rest of the body. Understanding what happens during each phase helps explain everything from mood changes to fertility windows to why a period can sometimes show up late.
The Four Phases of the Cycle
The menstrual cycle has four distinct phases, each driven by different hormones and serving a specific purpose.
Menstruation
This is your period. The lining of the uterus sheds and flows out through the vagina as a mix of blood, mucus, and tissue. During this phase, the uterine lining is at its thinnest, roughly 1 to 4 millimeters. A typical period lasts anywhere from two to seven days.
The Follicular Phase
The follicular phase overlaps with menstruation and continues after bleeding stops. During this time, the pituitary gland (a small structure at the base of your brain) releases a hormone that stimulates the ovaries to grow between 3 and 30 small fluid-filled sacs called follicles. Each follicle contains an immature egg, but usually only one becomes dominant and matures fully. That dominant follicle starts producing estrogen, which signals the other follicles to break down and causes the uterine lining to rebuild. By the end of this phase, the lining thickens to about 12 to 13 millimeters.
Ovulation
Rising estrogen triggers a surge of luteinizing hormone from the pituitary gland. That surge causes the dominant follicle to rupture, releasing a mature egg from the ovary. The egg is released roughly 10 to 12 hours after the hormone surge peaks. This is the most fertile point in the cycle, with the highest chance of conception in the two to three days leading up to ovulation.
The Luteal Phase
After ovulation, the released egg travels through the fallopian tube toward the uterus. The empty follicle left behind on the ovary transforms into a structure that produces progesterone, a hormone that continues thickening the uterine lining in preparation for a fertilized egg. The lining reaches its maximum thickness of about 16 to 18 millimeters during this phase. If the egg isn’t fertilized, progesterone levels drop, the lining breaks down, and menstruation begins again.
How Hormones Drive the Cycle
Four hormones do most of the work. Two come from the pituitary gland in your brain: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These signal the ovaries to grow eggs and eventually release one. The other two, estrogen and progesterone, are produced by the ovaries themselves and act primarily on the uterus and breasts.
The interplay between these hormones creates a feedback loop. Early in the cycle, FSH stimulates follicle growth. The growing follicles produce estrogen, which thickens the uterine lining. When estrogen reaches a high enough level, it triggers the LH surge that causes ovulation. After ovulation, progesterone takes over, maintaining the uterine lining for about two weeks. If pregnancy doesn’t occur, both estrogen and progesterone drop sharply, and the cycle resets.
This hormonal rhythm also explains many of the physical and emotional shifts people experience throughout the month. Bloating, breast tenderness, mood changes, and energy fluctuations all track with these rising and falling hormone levels.
Signs of Ovulation
Your body gives off noticeable signals around ovulation. Cervical mucus changes are one of the most reliable: just before ovulation, mucus becomes noticeably more abundant, thin, and slippery (often compared to raw egg whites). Just after ovulation, it decreases in amount and becomes thicker.
Basal body temperature, your temperature when fully at rest, also shifts. In most people, it rises by 0.5 to 1°F during ovulation and stays elevated until the next period starts. The catch is that by the time your temperature rises, ovulation has already happened. The most fertile days are the two to three days before that temperature increase, which is why tracking over several months helps you spot the pattern.
When the Cycle Starts and Ends
Most people get their first period between ages 11 and 13. Data from a large study found that the average age has been trending earlier: people born between 1950 and 1969 had an average first period at 12.5 years old, while those born between 2000 and 2005 averaged 11.9 years. The reasons likely include changes in nutrition, body weight, and environmental factors.
Cycles continue until menopause, which typically occurs in the late 40s to early 50s. In the years leading up to menopause (a stage called perimenopause), cycles often become less predictable, with periods arriving closer together or farther apart before stopping entirely.
Common Causes of Irregular Cycles
Plenty of things can knock a cycle off its usual rhythm. Some are temporary, like stress, significant weight changes, or travel. Others point to an underlying condition worth investigating.
- Polycystic ovary syndrome (PCOS): The ovaries produce excess androgens, a type of hormone that can prevent or delay ovulation. This leads to irregular or missed periods, and some people with PCOS stop menstruating entirely.
- Thyroid disorders: Both an underactive and overactive thyroid disrupt the hormonal signals that regulate the cycle, causing periods to become unpredictable.
- Primary ovarian insufficiency: The ovaries stop functioning normally before age 40, leading to missed or irregular periods. It can occur on its own or as a side effect of certain medical treatments.
- Endometriosis: Tissue similar to the uterine lining grows outside the uterus, often attaching to the ovaries or fallopian tubes, and can affect cycle regularity and cause significant pain.
Medications, including hormonal birth control, can also change cycle patterns. Some methods are designed to lighten, shorten, or eliminate periods altogether.
What Counts as Heavy Bleeding
Normal blood loss during a period is under 60 milliliters, roughly four tablespoons, over the entire period. Losses between 60 and 100 milliliters are considered moderately heavy, and anything over 80 to 100 milliliters qualifies as excessive. In practice, most people don’t measure their blood loss directly. Signs that bleeding may be heavier than normal include soaking through a pad or tampon every hour for several hours, passing blood clots larger than a quarter, or needing to change protection overnight.
Why Tracking Your Cycle Matters
Researchers at Harvard now describe the menstrual cycle as a vital sign, on par with heart rate, blood pressure, and temperature. Tracking your cycle length, flow, and symptoms over time can reveal patterns that signal health changes before other tests catch them. A cycle that suddenly becomes much shorter, longer, or more painful can be an early clue to thyroid problems, PCOS, or other hormonal conditions.
You don’t need anything fancy to start. Writing down the first day of each period and noting any symptoms is enough to build a useful record. Many apps automate this, but a simple calendar works just as well. Having several months of data gives you a personal baseline, which makes it much easier to spot when something has changed.