The menstrual cycle is a repeating hormonal process that prepares the body for pregnancy each month. A typical cycle lasts 21 to 35 days, counted from the first day of one period to the first day of the next. During that time, hormones rise and fall in a coordinated sequence that thickens the uterine lining, releases an egg, and either supports a fertilized egg or sheds the lining as a period.
How the Cycle Is Controlled
The menstrual cycle is driven by a communication loop between three structures: a region of the brain called the hypothalamus, the pituitary gland just below it, and the ovaries. The hypothalamus releases a signaling hormone that tells the pituitary to produce two key hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH prompts the ovaries to begin maturing an egg inside a fluid-filled sac called a follicle. As the follicle grows, it produces estrogen. When estrogen and progesterone levels rise high enough, they signal back to the brain to slow production of FSH and LH. This feedback loop is what keeps the cycle moving in a predictable rhythm, resetting itself each month when hormone levels drop.
The Follicular Phase
The cycle begins on the first day of your period. This marks the start of the follicular phase, which lasts from the onset of menstruation until ovulation. During the early days, FSH levels climb, stimulating several follicles in the ovaries to start developing. One follicle eventually becomes dominant and produces increasing amounts of estrogen.
That rising estrogen does two things. First, it rebuilds the uterine lining (the endometrium) that was just shed during your period. The lining takes on a layered structure and thickens to roughly 12 to 13 millimeters by the time ovulation approaches. Second, it triggers a shift in LH levels. LH stays low early on, then begins rising in the mid-follicular phase as estrogen climbs.
Ovulation
Ovulation is the release of a mature egg from the dominant follicle. It happens roughly 10 to 12 hours after LH hits its peak, an event often called the “LH surge.” This surge is set off by a sharp rise in estrogen from the nearly mature follicle. Once released, the egg travels into the fallopian tube, where it can be fertilized for roughly 12 to 24 hours.
Your body gives some outward signals around ovulation. Cervical mucus becomes clear, stretchy, and slippery in the days leading up to egg release, a response to high estrogen without progesterone. This type of mucus helps sperm travel more easily. Basal body temperature, your resting temperature taken first thing in the morning, also shifts slightly after ovulation, rising by less than half a degree Fahrenheit (about 0.3°C). When that small increase holds steady for three or more days, ovulation has likely occurred.
The Luteal Phase
After the egg is released, the empty follicle transforms into a temporary hormone-producing structure called the corpus luteum. Its main job is to secrete progesterone, which further prepares the uterine lining for a potential pregnancy. During this phase the lining reaches 16 to 18 millimeters and becomes more spongy and nutrient-rich. Cervical mucus dries up noticeably as progesterone rises and estrogen falls.
The luteal phase lasts about 14 days in most people and is the more consistent half of the cycle. Variations in total cycle length are usually caused by a shorter or longer follicular phase, not by changes in the luteal phase. If the egg is not fertilized, the corpus luteum breaks down, progesterone and estrogen levels drop sharply, and the cycle resets.
Menstruation
That hormone withdrawal is what triggers your period. As progesterone falls, the blood vessels supplying the uterine lining constrict and coil, cutting off blood flow to the top layers of tissue. The uterus releases compounds called prostaglandins that cause the muscular wall to contract, helping shed the degraded tissue. These contractions are what you feel as cramps.
A typical period lasts 2 to 7 days. Total blood loss averages about 60 milliliters per cycle, which is roughly 4 tablespoons. Losing more than 80 milliliters regularly is considered heavy menstrual bleeding. Signs of heavy bleeding include soaking through a pad or tampon every hour for several consecutive hours, needing to double up on protection, or passing blood clots larger than a quarter.
What Counts as a Normal Cycle
A healthy adult cycle falls between 21 and 35 days, and some variation from month to month is completely normal. A cycle that’s consistently 28 days is often cited as the “average,” but plenty of people run shorter or longer without anything being wrong. What matters more than hitting a specific number is whether your cycle is relatively predictable for you.
Tracking your cycle for a few months gives you a personal baseline. You can note the start date of each period, how many days bleeding lasts, and any symptoms like cramping, mood shifts, or changes in cervical mucus. That pattern becomes a useful reference point. The American College of Obstetricians and Gynecologists considers the menstrual cycle an additional vital sign, meaning changes in your cycle can be early clues to broader health shifts.
Cycles During Adolescence
Periods are often irregular for the first few years after they start. The hormonal feedback loop takes time to mature, so it’s common for teens to have cycles that are longer, shorter, or unpredictable from month to month. By the third year after the first period, 60 to 80 percent of cycles fall into the typical 21 to 34 day adult range. Cycles that remain very irregular beyond that point are worth discussing with a healthcare provider, since they can reflect hormonal conditions that are easier to manage when caught early.
How the Cycle Changes Before Menopause
In the years leading up to menopause, a transition called perimenopause, cycle patterns shift again. About 62 percent of people follow a relatively stable cycle length right up until their final period. But roughly 22 percent experience cycles that start getting noticeably longer within the last two years before menopause, and about 16 percent see that lengthening begin as early as five years before their final period. Very long cycles, skipped periods, and heavier or lighter flow all become more common during this window as the ovaries produce less consistent amounts of estrogen and progesterone.
These changes are a normal part of reproductive aging, though they can overlap with symptoms like hot flashes, sleep disruption, and mood changes. Keeping a record of your cycle during this time helps you and your provider distinguish expected perimenopause patterns from changes that deserve a closer look.