Menstruation is part of the follicular phase, which is the first phase of the menstrual cycle. More specifically, your period marks the very beginning of both the follicular phase and the entire cycle. Day one of your period is day one of your cycle.
That answer can be slightly confusing because some sources break the cycle into two phases (follicular and luteal), while others break it into four (menstrual, follicular, ovulation, luteal). Both frameworks are correct. The difference is just how finely you slice it. In the two-phase model, menstruation is the opening act of the follicular phase. In the four-phase model, it gets its own dedicated stage. Either way, it comes first.
How the Menstrual Cycle Is Structured
A menstrual cycle is measured from the first day of your period to the day before your next period. The average cycle runs 21 to 35 days, though it varies from person to person and even cycle to cycle.
In the simplest breakdown, the cycle has two main halves separated by ovulation:
- Follicular phase (days 1 through ~14): Starts on the first day of your period and lasts about 13 to 14 days. During this time, the brain signals the ovaries to develop a mature egg, and the uterine lining begins rebuilding.
- Ovulation (~day 14): A mature egg is released from the ovary.
- Luteal phase (days ~15 through ~28): The body prepares the uterine lining for a possible pregnancy. If no fertilized egg implants, hormone levels drop and the cycle resets with your next period.
Menstruation sits at the front end of the follicular phase. The bleeding itself typically lasts 2 to 7 days, so once it ends, you’re still in the follicular phase for another week or so as your body prepares for ovulation.
What Triggers Your Period to Start
The drop in progesterone is the direct trigger for menstruation. Here’s what happens: during the second half of your cycle, the ovary produces a temporary structure called the corpus luteum, which pumps out progesterone and estrogen to maintain the uterine lining. If no pregnancy occurs, that structure breaks down about 14 days after ovulation, and hormone levels fall sharply. That withdrawal, especially of progesterone, tells the body it’s time to shed the lining.
Once progesterone drops, a cascade of changes hits the uterine lining. Blood vessels in the lining become fragile and permeable, the tissue swells, and immune cells flood the area. Specialized spiral arteries in the lining constrict, temporarily cutting off blood supply to the upper layer. The oxygen deprivation that follows actually plays a functional role: brief, controlled hypoxia helps initiate the breakdown and is later required for normal tissue repair.
What Happens Inside the Uterus During Your Period
The uterine lining has two layers. The deeper base layer stays intact throughout the cycle. The upper functional layer is the one that thickens each month and gets shed during menstruation.
As progesterone drops, enzymes activate in targeted zones of the functional layer and begin dissolving the tissue. The lining breaks apart, and the mix of blood, tissue, and fluid exits through the cervix and vagina. Most people lose about 30 to 50 milliliters of blood per period, roughly two to three tablespoons. Losing more than 80 milliliters per cycle is considered clinically heavy bleeding.
Even while the lining is still shedding, repair is already underway. New cells grow from the base of tiny glands in the remaining layer, spreading outward until they meet cells from neighboring glands and form a fresh surface. This process involves resolving the inflammation, growing new blood vessels, and rebuilding tissue. It’s one of the few places in the body where this kind of rapid, scar-free healing happens on a regular cycle.
Why Cramps and Other Symptoms Happen
Your uterus is a muscle, and it contracts during your period to help push out the shedding lining. Those contractions are driven by prostaglandins, hormone-like compounds that ramp up inflammation and pain signaling. The higher your prostaglandin levels, the stronger the contractions, and the worse the cramping tends to be.
Cramps typically start one to three days before bleeding begins, peak about 24 hours into your period, and ease up within two to three days. The pain is usually centered in the lower abdomen but can radiate to the lower back and thighs. Some people also experience nausea, loose stools, headaches, or dizziness during this window, all of which are linked to the same inflammatory compounds circulating in the body.
How Menstruation Connects to the Rest of the Cycle
Even though menstruation feels like an ending, hormonally it’s a beginning. While you’re still bleeding, your brain is already releasing signals that tell the ovaries to start developing a new batch of follicles, one of which will eventually release a mature egg at ovulation. Estrogen levels start climbing within the first few days of your period, which is what stimulates the uterine lining to start thickening again.
This overlap is why menstruation belongs to the follicular phase rather than standing entirely on its own. The shedding of last cycle’s lining and the building of the next cycle’s lining happen almost simultaneously. By the time your period ends, the new lining is already several days into its growth, and the follicle that will ovulate is well on its way to maturing.