Your period is the menstruation phase, the first of four phases in the menstrual cycle. It marks Day 1 of a new cycle and typically lasts 3 to 7 days. The other three phases that follow are the follicular phase, ovulation, and the luteal phase.
The Four Phases in Order
The menstrual cycle moves through a repeating sequence of four distinct phases:
- Menstruation (your period): The uterine lining sheds and exits the body as a mix of blood, mucus, and tissue cells. This is Day 1 through roughly Day 5 to 7.
- Follicular phase: Starts on the first day of your period and lasts about 13 to 14 days total, overlapping with menstruation. During this stretch, the ovaries begin developing small fluid-filled sacs (follicles), each containing an immature egg.
- Ovulation: A mature egg releases from one ovary, usually about two weeks before your next period. This is the fertile window.
- Luteal phase: After the egg is released, the uterine lining thickens in preparation for a possible pregnancy. If the egg isn’t fertilized, hormone levels drop and the cycle resets with another period.
The average full cycle is 28 days, though anything from 21 to 35 days is normal for most adults. In teens, cycles can range from 21 to 45 days as the body is still regulating.
What Happens in Your Body During Your Period
The period exists because pregnancy didn’t happen in the previous cycle. When no fertilized egg implants, levels of estrogen and progesterone drop sharply. That hormone withdrawal signals the uterine lining that it’s no longer needed, triggering the shedding process.
The actual shedding relies on chemical messengers called prostaglandins, which cause the uterus to contract. Those contractions squeeze the lining away from the uterine wall and push it out. This is also why cramps happen: higher-than-normal prostaglandin levels lead to stronger, more painful contractions. The same mechanism explains why some people experience heavy periods alongside intense cramping.
Even while you’re bleeding, your body is already preparing for the next cycle. The follicular phase overlaps with menstruation, so the ovaries start recruiting new follicles during your period. Your body doesn’t wait for bleeding to stop before moving forward.
How to Identify Day 1
Day 1 of your cycle is the first day of actual bleeding, not spotting. Light spotting before a full flow can sometimes appear a day or two early, but the cycle clock starts when you see consistent red blood. Cleveland Clinic recommends marking each bleeding day on a calendar: the first mark is Day 1, and when bleeding stops, you stop marking. The next time bleeding begins again, that’s Day 1 of a brand new cycle.
This distinction matters if you’re tracking fertility, using cycle-based birth control, or monitoring cycle regularity. Counting spotting as Day 1 can throw off your calculations by a day or two.
Common Symptoms During Menstruation
The rapid hormone shift at the start of your period triggers a recognizable set of physical and emotional symptoms. On the physical side, the most common are cramps in the lower belly or back, bloating, headaches, tender breasts, muscle aches, poor sleep, and low energy. Emotionally, mood swings and feeling easily sad or irritated are typical.
Your basal body temperature also reflects which phase you’re in. During menstruation and the rest of the pre-ovulation window, body temperature tends to sit between 96 and 98°F (35.5 to 36.6°C). After ovulation, it rises to 97 to 99°F (36.1 to 37.2°C) and stays elevated through the luteal phase. If you’re tracking your temperature, you’ll notice it drops back down around the time your period starts.
What’s Normal and What’s Not
Most periods produce less than 60 milliliters of blood, roughly 4 tablespoons spread across the entire period. Flow between 60 and 100 mL is considered moderately heavy, and anything above 80 to 100 mL falls into the heavy bleeding category. Since measuring exact volume isn’t practical, soaking through a pad or tampon every hour for several consecutive hours is a useful real-world signal of unusually heavy flow.
Between 14% and 25% of women experience irregular cycles, meaning periods that are shorter or longer than usual, heavier or lighter, or accompanied by significant pain. Irregular cycles can still include ovulation, or they may be anovulatory, where no egg is released at all. Causes range widely: stress, significant changes in exercise or diet, smoking, alcohol, and body weight fluctuations can all shift hormone levels enough to alter your period. When periods disappear entirely for three months or more (and pregnancy isn’t the reason), the medical term is amenorrhea.
Bleeding or spotting between periods is considered irregular and is separate from your actual menstrual phase. If it happens occasionally, it’s usually not a concern, but recurring mid-cycle bleeding is worth investigating.