The menstrual cycle follows four phases in order: menstruation, the follicular phase, ovulation, and the luteal phase. A full cycle typically lasts 21 to 35 days, with 28 days being the most commonly cited average, though only about 16% of women actually have a median cycle length of exactly 28 days. Each phase is driven by shifting hormone levels that prepare the body for a potential pregnancy, then reset when pregnancy doesn’t occur.
Phase 1: Menstruation
The cycle begins with your period. This is when the uterine lining sheds because it wasn’t needed for a pregnancy. Bleeding typically lasts 3 to 7 days. Day 1 of your period is considered Day 1 of your entire cycle.
What triggers this shedding is straightforward: a small structure on the ovary called the corpus luteum, left over from the previous cycle, stops producing progesterone when no fertilized egg implants. Without progesterone to maintain it, the thickened uterine lining breaks down and exits through the vagina.
Phase 2: The Follicular Phase
The follicular phase overlaps with menstruation. It starts on Day 1 of your period and lasts until ovulation, spanning roughly 14 to 21 days. This is the longest and most variable phase of the cycle, which is why cycle lengths differ so much from person to person and even month to month.
During this phase, the brain signals the ovaries to start developing small fluid-filled sacs called follicles, each containing an immature egg. Usually one follicle becomes dominant and continues growing while the others are reabsorbed. As this dominant follicle matures, it releases increasing amounts of estrogen, which does two things: it thickens the uterine lining (building a fresh nutrient-rich layer) and it eventually triggers the hormonal surge that causes ovulation.
If your cycle runs shorter than average, it’s almost always because your follicular phase is shorter. If your cycle is longer, this phase is the one stretching out.
Phase 3: Ovulation
Ovulation is brief, lasting roughly 12 to 24 hours. It happens when the mature follicle ruptures and releases an egg into the fallopian tube. This is triggered by a sharp spike in luteinizing hormone (LH), which is what at-home ovulation test strips detect.
In a 28-day cycle, ovulation typically occurs around Day 14, but this varies widely depending on how long your follicular phase lasts. Someone with a 21-day cycle may ovulate around Day 7, while someone with a 35-day cycle may not ovulate until Day 21. The egg survives for about 24 hours after release. If sperm are present in the fallopian tube during that window, fertilization can occur.
Your body gives a few physical signals around ovulation. Basal body temperature rises slightly, typically less than half a degree Fahrenheit, and stays elevated for three or more days afterward. Cervical mucus also changes, becoming clearer, slipperier, and more stretchy in the days leading up to ovulation.
Phase 4: The Luteal Phase
After ovulation, the empty follicle transforms into the corpus luteum, a temporary hormone-producing structure on the ovary. Its main job is pumping out progesterone, which stabilizes and maintains the uterine lining so a fertilized egg could implant.
Unlike the follicular phase, the luteal phase is relatively consistent. It averages 12 to 14 days, with a normal range of 10 to 17 days. This consistency is why, if you know when you ovulated, you can predict your next period with reasonable accuracy.
During this phase, progesterone dominates. It raises your basal body temperature (which is why tracking temperature can confirm ovulation after the fact), and it changes the uterine lining from a growing, building state to a secretory state, meaning the lining matures and prepares to receive an embryo. Many people notice physical changes during this phase: breast tenderness, bloating, mood shifts, and fatigue are all linked to elevated progesterone.
What Happens at the End of the Cycle
If the egg isn’t fertilized, the corpus luteum breaks down after about 10 to 14 days. As it degrades, progesterone and estrogen levels drop sharply. Without those hormones supporting it, the uterine lining can no longer sustain itself. It sheds, your period starts, and the cycle resets to Day 1.
If fertilization does occur, the developing embryo produces a hormone that keeps the corpus luteum alive and producing progesterone, preventing the lining from shedding. This is the hormone detected by pregnancy tests.
Two Cycles Happening at Once
One detail that clarifies the whole picture: the menstrual cycle is really two synchronized cycles running in parallel. The ovarian cycle (what’s happening on the ovaries) has a follicular phase and a luteal phase, separated by ovulation. The uterine cycle (what’s happening to the lining of the uterus) has a menstrual phase, a proliferative phase where the lining thickens, and a secretory phase where the lining matures.
These map onto each other directly. The ovarian follicular phase corresponds to the uterine proliferative phase, because rising estrogen from the growing follicle is what thickens the lining. The ovarian luteal phase corresponds to the uterine secretory phase, because progesterone from the corpus luteum is what matures the lining. When people talk about “four phases,” they’re combining both cycles into a single timeline.
Why Cycle Length Varies
A study tracking over 1.5 million women found that about 91% had a usual cycle length between 21 and 35 days. That’s a 14-day range of normal. Most of this variation comes from the follicular phase, which can run anywhere from 14 to 21 days. Stress, illness, weight changes, travel, and age all influence how quickly (or slowly) a follicle matures, which shifts when ovulation happens and therefore how long the overall cycle lasts.
The luteal phase, by contrast, stays relatively stable for a given person. So if your cycles are irregular, the issue is almost always in the first half, not the second.