The first phase of the menstrual cycle is the follicular phase, which begins on day 1 of your period and lasts until ovulation. Because menstruation (the days you actively bleed) falls within the follicular phase, it’s sometimes described as its own distinct stage. But in clinical terms, the cycle is divided into two main phases, and the follicular phase comes first.
Why the Naming Can Be Confusing
Depending on the source, you’ll see the first phase called the “follicular phase,” the “proliferative phase,” or simply the “menstrual phase.” These aren’t competing definitions. The follicular phase spans from the first day of bleeding all the way to ovulation, typically 10 to 16 days total. Menstruation, the 3 to 7 days of active bleeding, is the opening act of that larger phase. Some health resources break the cycle into four stages (menstrual, follicular, ovulation, luteal) for simplicity, but the medical standard uses two: follicular and luteal.
The follicular phase gets its name from what’s happening in the ovaries during this time. Small fluid-filled sacs called follicles are developing, each containing an egg. By the end of the phase, one dominant follicle will release its egg at ovulation, marking the transition into the luteal phase.
What Triggers Your Period to Start
The signal that kicks off the first phase is a sharp drop in two hormones: estrogen and progesterone. These hormones had been sustaining the thickened lining of your uterus during the previous cycle’s luteal phase. When no pregnancy occurs, the tissue that produces them breaks down, and their levels plummet. That withdrawal is what sets menstruation in motion.
The drop in progesterone triggers changes in the small, coiled blood vessels (called spiral arteries) embedded in the uterine lining. These arteries constrict tightly, cutting off blood flow to the upper layers of tissue. After a period of intense constriction, the vessels suddenly relax and open back up. The combination of oxygen deprivation and the rush of returning blood flow causes the tissue to break apart and shed. Chemical messengers called prostaglandins drive the uterine contractions that push this tissue out as menstrual flow.
What Happens During Bleeding
The typical period lasts 3 to 7 days, with anything longer than that considered prolonged. Total blood loss averages about 60 milliliters per cycle, roughly 2.7 ounces, or about 4 tablespoons. Losing more than 80 milliliters consistently is classified as heavy menstrual bleeding. In practice, that can look like soaking through a pad or tampon every hour for several hours, passing blood clots larger than a quarter, or needing to change protection overnight.
Heavy bleeding isn’t just defined by volume, though. The UK’s National Institute for Health and Care Excellence broadened the definition to include any menstrual blood loss that interferes with your physical, emotional, or social quality of life, even if the actual volume is under 80 milliliters. If your period regularly disrupts your daily routine, that’s worth bringing up regardless of how much blood you think you’re losing.
Why Cramps and Digestive Issues Happen
Prostaglandins are the reason your period comes with side effects beyond bleeding. These molecules trigger the uterine contractions needed to shed the lining, but they don’t stay neatly confined to the uterus. When prostaglandin levels run high, they can act on smooth muscle throughout your body, including your gastrointestinal tract. That’s why many people experience loose stools, nausea, or bloating during the first few days of their period. Excess prostaglandins are also the primary driver of painful menstrual cramps (dysmenorrhea).
Over-the-counter anti-inflammatory pain relievers work for cramps precisely because they block prostaglandin production. Taking them at the very start of bleeding, or even just before your period begins, tends to be more effective than waiting until pain builds.
Your Body Is Already Preparing for the Next Egg
Here’s what surprises most people: while you’re still bleeding, your body has already started preparing for the next ovulation. The hormone that recruits new follicles in the ovaries, FSH (follicle-stimulating hormone), doesn’t wait for your period to end. Research tracking the precise timing found that FSH begins rising about 4 days before menses even starts, triggered by the same drop in estrogen that eventually causes bleeding. By the time you’re on day 2 or 3 of your period, FSH is actively stimulating a fresh batch of follicles to grow.
This overlap is why the follicular phase and the menstrual phase aren’t really separate events. Shedding and rebuilding happen simultaneously. Even as old tissue leaves the body, rising estrogen from the developing follicles will soon start thickening a brand-new uterine lining, transitioning you from the bleeding days into the proliferative portion of the follicular phase, when the lining regenerates in preparation for a possible pregnancy.