The menstrual cycle is a repeating sequence of hormonal shifts that prepares the body for pregnancy each month. A typical cycle lasts about 28 days, though anything from 21 to 35 days is normal. During that window, four hormones rise and fall in a coordinated pattern, driving changes in the ovaries, the uterine lining, body temperature, mood, and even the consistency of cervical fluid. Here’s what’s actually happening in each phase.
Phase 1: Menstruation
The cycle starts on the first day of bleeding. Progesterone withdrawal is the direct trigger: when no pregnancy occurs, the structure that was producing progesterone (more on that below) breaks down, hormone levels plummet, and the thickened uterine lining loses its support. The lining sheds, and that tissue and blood leave the body through the vagina.
Bleeding typically lasts two to seven days. Most people lose about two to three tablespoons of blood over the entire period, though it can feel like more because the flow also contains tissue and fluid. A sign of unusually heavy bleeding is soaking through more than one pad or tampon every hour or two. Cramps are common during this phase because the uterus contracts to help expel the lining.
Phase 2: The Follicular Phase
The follicular phase overlaps with menstruation at first, then continues after bleeding stops. It runs from day one of your period until ovulation, roughly the first 14 days of a 28-day cycle (though this varies widely between people and even between cycles).
During this phase, the brain’s pituitary gland releases follicle-stimulating hormone (FSH). FSH does exactly what its name suggests: it stimulates several small follicles in the ovaries to start growing. Each follicle contains an immature egg. Over the course of about a week, one follicle outcompetes the others and becomes the “dominant” follicle. The rest stop developing and are reabsorbed. This selection process is driven by complex signaling between the follicles and FSH. Follicles that respond most strongly to FSH survive, while those with weaker responses undergo a natural cell-death process.
As the dominant follicle grows, it produces rising amounts of estrogen. That estrogen has two important jobs. First, it signals the uterine lining to start rebuilding, growing thicker and developing a rich blood supply. Second, it feeds back to the brain, gradually shifting the hormonal signals that will trigger the next phase.
This is also when many people feel their best during the cycle. Energy tends to be higher, and mood often improves as estrogen climbs. Cervical fluid, which is dry or tacky right after your period, gradually becomes wetter and creamier as the follicular phase progresses.
Phase 3: Ovulation
Ovulation is the shortest phase, lasting only about 24 to 36 hours. When estrogen from the dominant follicle reaches a critical threshold, the brain responds with a sudden surge of luteinizing hormone (LH). This LH surge triggers the mature follicle to rupture and release its egg into the fallopian tube.
The egg is viable for about 12 to 24 hours after release. Because sperm can survive in the reproductive tract for up to five days, the fertile window extends from roughly five days before ovulation through the day of ovulation itself.
Your body gives a few observable signals around ovulation. Cervical fluid becomes its most slippery and stretchy during the days leading up to egg release, resembling raw egg whites. This consistency helps sperm travel more easily. After ovulation, basal body temperature (your temperature at complete rest) rises by a small but measurable amount, typically less than half a degree Fahrenheit, though the increase can range from 0.4°F to 1°F. That temperature stays elevated for the rest of the cycle.
Phase 4: The Luteal Phase
After the egg is released, the empty follicle left behind in the ovary transforms into a temporary structure called the corpus luteum. This structure acts like a small hormone factory, producing large amounts of progesterone along with some estrogen. The luteal phase lasts roughly 10 to 16 days and is more consistent in length than the follicular phase.
Progesterone is the dominant hormone here, and its primary role is to prepare the uterine lining for a potential pregnancy. It makes the lining softer, more nutrient-rich, and better supplied with blood vessels, essentially creating an environment where a fertilized egg could implant and grow. Progesterone also raises body temperature (which is why the post-ovulation temperature shift persists) and thickens cervical fluid, making it drier and less penetrable.
If a fertilized egg implants in the uterine lining, it starts producing signals that keep the corpus luteum alive and producing progesterone through the first trimester of pregnancy. If no implantation occurs, the corpus luteum degrades after about 10 to 14 days. Progesterone and estrogen levels drop sharply, the uterine lining can no longer sustain itself, and the cycle resets with menstruation.
Why PMS Happens in the Luteal Phase
Premenstrual symptoms like bloating, breast tenderness, irritability, and mood changes are tied directly to progesterone’s behavior in the second half of the cycle. Progesterone rises rapidly after ovulation and stays elevated before falling in the days before your period. But the pattern of that decline appears to matter. Research published in Psychoneuroendocrinology found that people who developed PMS symptoms had progesterone levels that stayed stable through most of the late luteal phase and then dropped sharply in the final three days before menstruation. People without symptoms experienced a more gradual decline over the last eight days.
The connection between hormones and mood runs through the brain’s chemical messengers. Estrogen and progesterone cross into the brain readily and influence the activity of serotonin, dopamine, and GABA, three neurotransmitters that regulate mood, motivation, and anxiety. Progesterone gets converted into a compound called allopregnanolone, which enhances the calming effects of GABA. When progesterone drops suddenly before a period, that calming influence drops with it, which may explain the anxiety and irritability many people experience in the days before bleeding starts.
Tracking Your Own Cycle
Two of the simplest things to track are cervical fluid and cycle length. On a 28-day cycle, cervical fluid follows a fairly predictable progression: dry or tacky in the days right after your period, then sticky, then creamy and wet around days seven to nine, then stretchy and slippery (peak fertility) around days 10 to 14, then dry again after ovulation until the next period. Noting these changes day by day can help you identify when you’re ovulating, even if your cycle doesn’t follow a textbook 28-day pattern.
Basal body temperature is another useful marker, though it confirms ovulation after the fact rather than predicting it. You need a thermometer that reads to a tenth of a degree and should take your temperature first thing in the morning before getting out of bed. Once you see the temperature shift and it stays elevated for at least three days, ovulation has likely occurred.
Signs of an Irregular Cycle
Some variation from cycle to cycle is completely normal, especially during the first few years of menstruation and in the years leading up to menopause. But certain patterns fall outside the normal range. Cycles shorter than 21 days or longer than 35 days, bleeding that lasts longer than seven days, or bleeding so heavy it soaks through a pad or tampon every hour or two all qualify as abnormal uterine bleeding.
Heavy bleeding from the very first period onward can sometimes point to an underlying issue with how the blood clots. Screening questions include whether you bruise easily (one to two times per month), get frequent nosebleeds, have bleeding gums, or have a family history of bleeding problems. Having two or more of these alongside consistently heavy periods is worth investigating. Cycles that simply disappear for three months or more (outside of pregnancy) also signal that something in the hormonal chain has been disrupted, whether from stress, significant weight change, thyroid issues, or conditions like polycystic ovary syndrome.