The menstrual cycle is a repeating sequence of hormonal changes that prepares your body for pregnancy roughly once a month. A typical cycle lasts between 24 and 38 days, counted from the first day of one period to the first day of the next. The whole process is driven by a conversation between your brain and your ovaries, with hormones acting as the messengers that coordinate every step.
The Hormones Running the Show
Four hormones do most of the work. Your pituitary gland, a pea-sized structure at the base of your brain, releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Your ovaries respond by producing estrogen and progesterone. These four hormones rise and fall in a carefully timed pattern, and each shift triggers the next stage of the cycle. When any one of them is too high or too low at the wrong time, the whole sequence can be disrupted.
Phase 1: Menstruation
The cycle begins on the first day of your period. This is when the uterine lining, which thickened over the previous weeks, sheds because it wasn’t needed for a pregnancy. The trigger is simple: levels of both estrogen and progesterone drop sharply, and without those hormones supporting the lining, it breaks down and exits through the vagina as menstrual blood.
Most people bleed for three to seven days. The lining goes from its peak thickness of 16 to 18 millimeters (roughly three-quarters of an inch) down to almost nothing. While this shedding is happening, the next phase is already quietly starting in the background.
Phase 2: The Follicular Phase
The follicular phase overlaps with menstruation at first, then continues after bleeding stops. It lasts from day one of your period until ovulation, which means it can run anywhere from about 10 to 16 days depending on the person and the cycle.
Here’s what’s happening inside: your pituitary gland ramps up FSH, which travels through your bloodstream to your ovaries. FSH stimulates several small fluid-filled sacs called follicles to start growing. Each follicle contains an immature egg. Over the next several days, one follicle outpaces the others and becomes the “dominant” follicle. The rest stop developing and are reabsorbed.
As the dominant follicle grows, it produces increasing amounts of estrogen. That rising estrogen does two important things. First, it signals your uterus to start rebuilding its lining, thickening it to around 12 to 13 millimeters by the end of this phase. Second, once estrogen reaches a high enough level, it triggers a dramatic hormonal event that launches the next phase.
Phase 3: Ovulation
The spike in estrogen from the dominant follicle tells your pituitary gland to release a sudden burst of LH. This LH surge begins roughly 36 hours before ovulation, and the egg is released from the follicle about 8 to 20 hours after LH peaks. For most people with a 28-day cycle, ovulation happens around day 14, but it can vary widely.
Once the egg leaves the ovary, it’s swept into the fallopian tube by tiny hair-like structures. The egg is viable for only 12 to 24 hours after release. If sperm are present in the fallopian tube during that narrow window, fertilization can occur. If not, the egg disintegrates. Ovulation is the shortest phase, essentially a single event rather than a sustained period of time.
Phase 4: The Luteal Phase
After the egg is released, the empty follicle left behind on the ovary transforms into a temporary structure called the corpus luteum. This structure has one primary job: pumping out progesterone. Progesterone is the hormone that shifts the uterine lining from simply growing thicker to becoming a spongy, blood-vessel-rich environment that could support a fertilized egg. The lining reaches its maximum thickness of 16 to 18 millimeters during this phase.
The luteal phase lasts about 14 days in most people and is the most consistent part of the cycle. If a fertilized egg implants in the uterine lining, the embryo produces a hormone (HCG, the same one pregnancy tests detect) that keeps the corpus luteum alive and producing progesterone. If no implantation occurs, the corpus luteum breaks down after about two weeks. Progesterone and estrogen levels plummet, the thickened lining loses its hormonal support, and menstruation begins. The cycle starts over.
Changes You Can Actually Feel
Your body gives you visible and measurable signals as hormones shift throughout the cycle. One of the most noticeable is cervical mucus. In the days right after your period, discharge tends to be dry or tacky and white or yellowish. As you move toward ovulation and estrogen rises, mucus becomes creamy, then progressively wetter and more slippery. At peak fertility, around days 10 to 14 of a 28-day cycle, it stretches between your fingers like raw egg whites and feels distinctly wet. After ovulation, progesterone dries things up again, and mucus stays thick or minimal until your next period.
Your core body temperature also shifts. After ovulation, the progesterone released by the corpus luteum causes your resting (basal) body temperature to rise slightly, typically less than half a degree Fahrenheit (0.3°C). It stays elevated throughout the luteal phase and drops back down when your period arrives. This is why temperature tracking can help identify whether and when ovulation occurred, though the shift is only visible in hindsight since the rise happens after the egg is already released.
What Counts as a Normal Cycle
Cycles between 21 and 35 days are considered within the normal range, and bleeding that lasts 2 to 7 days is typical. But “normal” is somewhat personal. Your cycle might consistently run 25 days while someone else’s runs 33, and both are healthy. What matters more than hitting exactly 28 days is consistency. If your cycle length swings dramatically from month to month, or if you regularly fall outside the 21-to-35-day window, that can signal a hormonal imbalance worth investigating.
Cycles are also rarely identical across your lifetime. They tend to be more irregular in the first few years after menstruation starts and again in the years approaching menopause. Stress, significant weight changes, intense exercise, and illness can all shift your cycle temporarily by delaying ovulation, which lengthens the follicular phase while the luteal phase usually stays close to its standard 14 days.