The period cycle, or menstrual cycle, is a roughly monthly process in which your body prepares for pregnancy and then resets when pregnancy doesn’t occur. A typical cycle lasts about 28 days, though anything between 21 and 35 days is normal. It’s divided into four phases, each driven by shifting hormones that affect everything from your energy levels to your body temperature.
The Four Phases at a Glance
Your cycle starts on the first day of your period and ends the day before your next one begins. In a 28-day cycle, the phases break down like this:
- Menstrual phase (days 1–5): Your uterine lining sheds, producing your period.
- Follicular phase (days 1–13): Overlaps with menstruation and continues as your body prepares an egg for release.
- Ovulation (around day 14): A mature egg is released from the ovary.
- Luteal phase (days 15–28): Your body either supports a fertilized egg or prepares to start over.
These timelines shift if your cycle is shorter or longer than 28 days. Ovulation doesn’t always land on day 14, and the follicular phase is the part that varies most from person to person.
The Menstrual Phase
Your period marks the beginning of a new cycle. Over the previous weeks, your uterine lining thickened with blood and nutrients in preparation for a potential pregnancy. When no fertilized egg implants, levels of estrogen and progesterone drop sharply, and that thick lining sheds a little at a time over several days.
Most periods last between 3 and 7 days. The total blood loss during a normal period is under 60 milliliters, which is roughly 4 tablespoons. Between 60 and 100 milliliters is considered moderately heavy, and anything over 80 to 100 milliliters qualifies as heavy menstrual bleeding. If you’re soaking through a pad or tampon every hour for several hours, that’s a sign of unusually heavy flow.
The Follicular Phase
This phase starts on day 1 (overlapping with your period) and runs until ovulation. Early on, your pituitary gland, a small structure at the base of your brain, begins releasing a hormone that stimulates your ovaries to grow a batch of 3 to 30 small fluid-filled sacs called follicles. Each follicle contains an immature egg.
Within days, one follicle outpaces the others and becomes the dominant follicle. It starts pumping out estrogen, which does two things: it signals the other follicles to stop growing and reabsorb into the body, and it triggers your uterine lining to start thickening again in preparation for a possible pregnancy. This rising estrogen is also why many people feel a boost in energy and mood during the second half of this phase, after their period ends.
Ovulation
When estrogen from the dominant follicle reaches a critical level, it triggers a sharp spike in luteinizing hormone from the pituitary gland. This surge causes the mature follicle to rupture and release its egg from the surface of the ovary. The egg then enters the fallopian tube, where it can potentially be fertilized.
The egg survives for only 12 to 24 hours after release. That sounds like a tiny window, but sperm can live inside the reproductive tract for up to 5 days. This means the total fertile window is about 6 days per cycle: the 5 days before ovulation plus the day of ovulation itself. Some people feel a twinge of pain on one side of their lower abdomen during ovulation, and body temperature rises slightly afterward.
The Luteal Phase
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. Its job is to produce progesterone, the hormone that makes your uterine lining thick, nutrient-rich, and ready to support an embryo. Progesterone also thickens cervical mucus, making it harder for bacteria to enter the uterus, and it raises your basal body temperature slightly, which stays elevated until your next period starts.
If the egg isn’t fertilized, the corpus luteum begins breaking down about 10 days after ovulation. As it degrades, progesterone and estrogen levels fall. Without those hormones sustaining it, the uterine lining can no longer maintain itself. This triggers shedding, your period begins, and a new cycle starts. The drop in progesterone during this phase is also what drives many premenstrual symptoms like bloating, mood changes, breast tenderness, and fatigue.
What Counts as a Normal Cycle
A 28-day cycle gets all the attention, but it’s just an average. Cycles anywhere from 21 to 35 days are considered normal in adults. What matters more than hitting a specific number is consistency. If your cycle length varies by more than about 9 days from month to month (say, 28 days one cycle, then 37 the next, then 29), that pattern is considered clinically irregular.
Cycles also shift over a lifetime. They tend to be longer and less predictable in the first few years after periods start, settle into a more regular pattern through the 20s and 30s, and become irregular again as menopause approaches. Stress, significant weight changes, intense exercise, and certain medical conditions can all push cycles outside the normal range.
How Hormones Drive the Whole Process
Four hormones do most of the work, and they operate in a feedback loop between your brain and ovaries. Follicle-stimulating hormone (FSH) kicks things off in the follicular phase by telling the ovaries to start growing follicles. As the dominant follicle produces more estrogen, that rising estrogen signals the brain to reduce FSH, which is what causes the other follicles to die off.
When estrogen peaks, it triggers the surge in luteinizing hormone (LH) that causes ovulation. After ovulation, progesterone takes over as the dominant hormone, maintaining the uterine lining and keeping the body in a “wait and see” state. If no pregnancy occurs, progesterone drops, the lining sheds, and FSH starts climbing again. The whole system resets, and the cycle begins once more.
This is why hormonal birth control works the way it does. By supplying steady levels of synthetic hormones, it prevents the natural surges and dips that trigger follicle growth, ovulation, and the rest of the cascade.