A woman’s menstrual cycle is a repeating hormonal sequence that prepares the body for pregnancy each month. The full cycle typically lasts 21 to 35 days, with 28 days as a common average, and involves a coordinated chain reaction between the brain and the ovaries. Understanding what happens in each phase helps make sense of the physical changes you experience throughout the month.
The Four Phases at a Glance
The cycle has four overlapping phases: menstruation, the follicular phase, ovulation, and the luteal phase. Each one is driven by shifts in four key hormones: follicle-stimulating hormone (FSH), estrogen, luteinizing hormone (LH), and progesterone. These hormones rise and fall in a specific pattern, and each shift triggers the next event in the sequence.
Day 1 of your cycle is the first day of your period. From there, the follicular phase carries you toward ovulation around the midpoint, and the luteal phase fills the second half. If pregnancy doesn’t occur, hormone levels drop, the uterine lining sheds, and the whole process resets.
Menstruation: The Reset
Menstruation is the most visible part of the cycle. It happens because progesterone and estrogen levels have fallen sharply at the end of the previous cycle, signaling the uterus to shed the lining it built up. Bleeding typically lasts about 4 to 5 days, and the total blood loss is small, roughly 2 to 3 tablespoons over the entire period. Some cycles are shorter or longer, with anywhere from 2 to 7 days considered normal.
During this time, cervical mucus is minimal. After bleeding tapers off, discharge tends to be dry or tacky, often white or slightly yellow-tinged.
The Follicular Phase: Building Up
The follicular phase starts on day 1 and runs until ovulation. It overlaps with menstruation at first, but the real action is happening inside the ovaries. Your brain’s pituitary gland releases FSH, which tells the ovaries to start developing follicles. These are tiny fluid-filled sacs, each containing an immature egg. Somewhere between 11 and 20 follicles begin developing in a given cycle, but only one will win the race.
That single dominant follicle grows faster than the rest and begins pumping out estrogen. Rising estrogen does two important things. First, it thickens the uterine lining with blood vessels and tissue, creating a cushion where a fertilized egg could implant. Second, it feeds back to the brain, gradually building the pituitary gland’s capacity to release a massive burst of LH later on. The pituitary’s LH capacity increases several-fold between the early and late follicular phase, and this buildup is what makes the mid-cycle surge possible.
You can actually observe this phase through cervical mucus changes. Around days 7 to 9 of a 28-day cycle, mucus shifts from sticky to creamy and yogurt-like. By days 10 to 14, as estrogen peaks, it becomes slippery, stretchy, and resembles raw egg whites. This wet, clear mucus makes it physically easier for sperm to travel through the cervix.
Ovulation: The Main Event
Ovulation is the shortest phase but the most consequential. When estrogen from the dominant follicle reaches a critical threshold, it triggers a sudden spike in LH from the pituitary gland. This LH surge is the direct trigger for ovulation. After blood levels of LH rise, the ovary releases its mature egg roughly 36 to 40 hours later. In a textbook 28-day cycle, this happens around day 14, but in practice it varies widely from person to person and even cycle to cycle.
Home ovulation test kits detect LH in urine. Once those tests turn positive, ovulation usually follows within 12 to 24 hours. The fertile window, when conception is most likely, includes the few days of egg-white cervical mucus leading up to ovulation and the day of ovulation itself. After release, the egg survives for a relatively short time, which is why timing matters so much for conception.
The Luteal Phase: Waiting for a Signal
After the egg is released, the empty follicle left behind in the ovary transforms into a temporary structure called the corpus luteum. Its primary job is producing progesterone, along with some estrogen. Progesterone takes over as the dominant hormone for the second half of the cycle, roughly days 15 through 28.
Progesterone changes the uterine lining from a growing layer into a mature, stable environment ready to support an embryo. It also raises your basal body temperature slightly, which is why temperature tracking can confirm that ovulation has already happened. Cervical mucus dries up during this phase, becoming thick or nearly absent.
If a fertilized egg implants in the uterine lining, it begins producing a hormone that tells the corpus luteum to keep going. The corpus luteum continues making progesterone to sustain the early pregnancy until the placenta takes over. If no implantation happens, the corpus luteum breaks down after about 10 to 14 days. Progesterone and estrogen levels plummet, the uterine lining loses its hormonal support, and menstruation begins. Day 1 again.
How the Brain and Ovaries Communicate
The entire cycle runs on a feedback loop between three players: the hypothalamus (a region deep in the brain), the pituitary gland (just below it), and the ovaries. The hypothalamus releases a signaling hormone that tells the pituitary to produce FSH and LH. The pituitary sends those hormones to the ovaries, and the ovaries respond by producing estrogen and progesterone, which in turn signal back to the brain.
For most of the cycle, this feedback is negative. Rising estrogen and progesterone tell the brain to ease off on FSH and LH, keeping things in balance. But there’s one dramatic exception: just before ovulation, high estrogen flips the signal from “slow down” to “go.” Instead of suppressing LH, it amplifies the pituitary’s response to the hypothalamus, creating the LH surge that triggers egg release. This positive feedback moment is the pivot point of the entire cycle.
This loop also explains why stress, significant weight changes, or intense exercise can disrupt your cycle. These factors affect the hypothalamus, which sits at the top of the chain. If the hypothalamus dials back its signals, the entire downstream sequence can stall.
What Counts as a Normal Cycle
A cycle length anywhere from 21 to 35 days falls within the normal range. Your cycle might be consistent from month to month or vary by several days, and both patterns are common. Cycles tend to be more irregular in the first few years after puberty and again in the years leading up to menopause, when hormonal patterns are shifting.
If your period hasn’t arrived by age 15 or within five years of the first signs of puberty, that’s considered primary amenorrhea. If you’ve been menstruating and then miss your period for three or more consecutive months, that’s secondary amenorrhea. The most common cause of a suddenly missed period is pregnancy. Breastfeeding and menopause are other normal reasons periods stop. Beyond those, conditions that affect the hypothalamus, pituitary, thyroid, or ovaries can interrupt the cycle.
Tracking Your Cycle
Your body gives you visible signals about where you are in the cycle. Cervical mucus is one of the most reliable. In a 28-day cycle, mucus typically follows a progression: dry or tacky after your period ends, then creamy and white, then wet and stretchy like raw egg whites as ovulation approaches. After ovulation, it dries up again until your next period. That slippery, egg-white phase lasts about three to four days and marks your most fertile window.
Basal body temperature, tracked first thing each morning before getting out of bed, shows a small but consistent rise after ovulation due to progesterone. The shift confirms ovulation after the fact rather than predicting it in advance. Combining mucus observation with temperature tracking gives you a fuller picture of your cycle’s timing than either method alone.