A period is the shedding of the uterine lining that happens when pregnancy doesn’t occur during a menstrual cycle. The full cycle typically repeats every 21 to 35 days, with bleeding lasting 2 to 7 days. Behind that bleeding is a carefully timed chain of hormonal signals that builds up the lining, releases an egg, waits for fertilization, and then resets when fertilization doesn’t happen.
The Four Phases of the Cycle
The menstrual cycle has four distinct phases, each driven by different hormones. They overlap and flow into each other, but understanding them separately makes the whole process click.
Menstruation (days 1 to ~5): This is the period itself. Without a pregnancy to sustain, hormone levels drop sharply, and the thickened uterine lining breaks down and exits through the vagina as a mix of blood, tissue, and mucus. Prostaglandins, chemicals produced in the uterine lining, cause the uterine muscles and blood vessels to contract so the tissue can be expelled. These contractions are what you feel as cramps. Prostaglandin levels are highest on the first day of bleeding, which is why cramps tend to be worst at the start and ease up as the lining sheds.
Follicular phase (days 1 to ~13): This phase actually starts on the same day as your period. The brain’s pituitary gland releases follicle-stimulating hormone (FSH), which tells a group of tiny follicles in the ovaries to start maturing. Each follicle contains an immature egg. Over about two weeks, one follicle outcompetes the rest and becomes the dominant one, growing larger and producing rising levels of estrogen. That estrogen does two important things: it rebuilds the uterine lining (thickening it with new blood vessels and glands) and it changes cervical mucus to become more fluid and sperm-friendly.
Ovulation (~day 14): When estrogen reaches a critical threshold, it flips a switch. Instead of suppressing the pituitary gland as it normally does, high estrogen triggers a massive surge of luteinizing hormone (LH), with LH levels jumping roughly tenfold. About 36 to 44 hours after this surge begins, the dominant follicle ruptures and releases a mature egg into the fallopian tube. The egg survives only 12 to 24 hours after release. But because sperm can live inside the uterus for three to five days, the total window where sex can lead to pregnancy stretches from about five days before ovulation to one day after.
Luteal phase (~days 15 to 28): After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. It pumps out progesterone, the dominant hormone of this phase. Progesterone prepares the uterine lining for a possible pregnancy by developing complex glands, storing glycogen for energy, and expanding the blood supply. It also thickens cervical mucus, making it harder for sperm to pass through, and raises your basal body temperature slightly. If no fertilized egg implants, the corpus luteum breaks down after about 12 to 14 days. Progesterone and estrogen plummet, the lining can no longer sustain itself, and a new period begins.
Why You Get Cramps and Other Symptoms
Period cramps are not just “in your head.” They have a direct physical cause. As the uterine lining breaks down, it releases prostaglandins that force the uterine muscle to contract, squeezing the tissue out. Those same contractions temporarily compress blood vessels feeding the uterus, briefly cutting off oxygen to the muscle. That oxygen deprivation is part of why cramps can feel so intense, similar to the ache of a muscle working too hard.
Other common symptoms trace back to the same hormonal shifts. The drop in estrogen and progesterone before your period can trigger headaches, mood changes, bloating, and breast tenderness. Once bleeding starts and prostaglandin levels fall, many of these symptoms begin to ease within a day or two.
Physical Signs That Track Your Cycle
Your body gives visible cues about where you are in your cycle. Cervical mucus changes are one of the most reliable. In the days after your period ends, you may notice little to no discharge. As estrogen rises in the lead-up to ovulation, mucus becomes increasingly wet, clear, stretchy, and slippery, sometimes compared to raw egg whites. This “peak type” mucus is the most fertile sign. After ovulation, progesterone causes an abrupt drop in mucus production, and what remains tends to be thick, sticky, or absent.
Basal body temperature follows a similar pattern. It stays relatively low during the first half of the cycle, then rises slightly (usually by about 0.2 to 0.5°C) after ovulation due to progesterone. The temperature stays elevated through the luteal phase and drops again right before or during your period. Ovulation prediction kits work differently: they detect the LH surge in urine, giving you roughly 36 hours of notice before ovulation occurs.
What Counts as a Normal Cycle
Cycles between 21 and 35 days are considered normal, with bleeding lasting 2 to 7 days. That’s a wide range, and your own cycle length can shift from month to month by a few days without anything being wrong. The most common variation happens in the follicular phase (the first half). The luteal phase tends to stay more consistent at around 12 to 14 days.
Heavy menstrual bleeding is clinically defined as losing 80 milliliters or more per cycle, roughly equivalent to five and a half tablespoons. In practice, that’s hard to measure, so a more useful guideline is whether your bleeding interferes with your daily life. Soaking through a pad or tampon every hour for several consecutive hours, passing large clots, or needing to double up on protection are signs your flow may be heavier than typical.
How Cycles Change With Age
Periods don’t behave the same way at 14 as they do at 25 or 45. In the first few years after a person’s first period, cycles are often irregular because the hormonal system is still maturing. Estrogen levels take roughly a year after the first period to settle into a regular cycling pattern, and progesterone can remain low or unpredictable for several years. This means skipped periods, varying cycle lengths, and lighter or heavier flows are all common during adolescence.
In the years leading up to menopause, typically starting in the early to mid-40s, a similar kind of hormonal turbulence returns. Estrogen levels become erratic and can actually spike higher than normal during early perimenopause, while progesterone declines gradually. FSH starts rising as the brain tries harder to stimulate aging ovaries. The result is cycles that may shorten, lengthen, or become unpredictable, sometimes with heavier bleeding. As perimenopause progresses, estrogen eventually drops and stays low, FSH climbs steeply, and periods space out further until they stop altogether.
Between these two transitions, most people experience their most predictable cycles during their 20s and 30s, though even during these years, stress, weight changes, illness, and exercise can shift timing by days or weeks.