Your period is the menstruation phase, which is the first phase of the menstrual cycle. It begins on day 1 of your cycle and typically lasts about 5 days, though anywhere from 2 to 7 days is normal. This is when the lining of your uterus sheds because no fertilized egg implanted during the previous cycle.
The Four Phases in Order
The menstrual cycle has four phases that repeat in sequence:
- Menstruation (your period): Days 1 through 2–7, when the uterine lining sheds.
- Follicular phase: Starts on day 1 of your period and lasts 13 to 14 days, overlapping with menstruation. During this time, an egg matures inside the ovary.
- Ovulation: A mature egg is released from the ovary, usually around day 14.
- Luteal phase: The final stretch, roughly days 15 through 28, when the body prepares the uterine lining for a potential pregnancy.
One detail that often confuses people: menstruation and the follicular phase overlap. The follicular phase technically starts on day 1 of your period and runs for about two weeks. So your period happens during the early part of the follicular phase. Some sources list them as separate phases, which is accurate in terms of what’s happening to the uterine lining versus the ovaries, but they share the same calendar days.
Why Bleeding Happens
Throughout the second half of your previous cycle (the luteal phase), the lining of the uterus thickens with fluids and nutrients designed to nourish an embryo. If no egg is fertilized, your estrogen and progesterone levels drop sharply. That drop in progesterone is the direct trigger for menstruation.
When progesterone falls, the body ramps up production of inflammatory compounds called prostaglandins. These cause the tiny blood vessels in the uterine lining to constrict, cutting off blood supply to the top layers. Without blood flow, the tissue breaks down and sheds. This is also why periods come with cramps: prostaglandins cause the uterine muscle to contract, helping push the tissue out. Higher prostaglandin levels generally mean more intense cramps.
After shedding, the uterine lining essentially behaves like a healing wound. It repairs itself rapidly over the course of a few days, and by the end of your period, rising estrogen levels are already prompting a fresh lining to build.
What’s Normal for Period Length and Flow
A full menstrual cycle (from the first day of one period to the first day of the next) ranges from 21 to 35 days in most adults. The average is about 28 days, but having a slightly shorter or longer cycle is common and not a sign of a problem. Average blood loss per period is around 30 milliliters, roughly two tablespoons.
If you’re soaking through a pad or tampon every 1 to 2 hours, or your period consistently lasts longer than 7 days, that’s considered excessive flow. Chronic blood loss above 80 milliliters per cycle can lead to anemia over time.
For teens, cycles take a few years to settle into a predictable rhythm. In the first years after a first period, cycles can range from 21 to 45 days. By the third year, 60 to 80 percent of cycles fall into the adult range of 21 to 34 days. Going more than 90 days between periods at any age is uncommon enough to warrant a check-in with a healthcare provider.
How Periods Change During Perimenopause
As you move into your 40s, ovulation becomes less predictable, and the menstruation phase can shift noticeably. Periods may arrive closer together or further apart, and the flow can swing from unusually light to unusually heavy from one month to the next. If the length of your cycle starts varying by seven or more days compared to your usual pattern, that’s often an early sign of perimenopause. Later in the transition, gaps of 60 days or more between periods are typical before they stop entirely.
Tracking Your Period Phase
Beyond simply marking calendar dates, your body gives a couple of signals that help confirm where you are in the cycle. Basal body temperature (your temperature first thing in the morning before getting out of bed) drops just before your period starts, then stays in the lower range of about 96 to 98°F throughout menstruation. It won’t rise noticeably until after ovulation, roughly two weeks later.
Cervical mucus is another marker, though it’s hard to distinguish during the period itself because of menstrual flow. In the days right after your period ends, discharge tends to be dry or tacky, usually white or slightly yellow. This dry phase signals that estrogen is still relatively low and the body hasn’t yet begun ramping up toward ovulation.