I

A rhythm counted from day one.

The menstrual cycle is the recurring sequence of hormonal and structural changes that runs between one period and the next. By convention it is counted from the first day of bleeding, which is day one, through to the day before the next period starts. That counting rule matters more than it sounds: almost every figure quoted about the female menstrual cycle, from phase lengths to the fertile window, is anchored to it.

Four hormones carry most of the sequence. Follicle-stimulating hormone and luteinising hormone are released from the pituitary gland in the brain. Estradiol, the principal form of estrogen during the reproductive years, and progesterone are produced mainly by the ovary. Their rise and fall across the month is what divides the cycle into recognisable stages at all.

Two structures respond to those hormones on different schedules. In the ovary, a cohort of follicles is recruited, one usually becomes dominant, and after it releases its egg the remaining structure becomes the corpus luteum. In the uterus, the endometrium thickens, then is shed if no pregnancy occurs. The ovarian sequence and the uterine sequence run at the same time and describe the same month from two angles.

The four phases most people mean by period cycle phases are a practical blend of the two: one uterine event, menstruation, and three ovarian ones, the follicular phase, ovulation, and the luteal phase. It is a useful description precisely because it maps onto something visible.

II

The menstrual phase, where counting begins.

Menstruation is the shedding of the endometrium, the lining the uterus built across the previous cycle in case a pregnancy occurred. When the corpus luteum from that cycle stops producing progesterone, the lining is no longer maintained, and it is released. Bleeding most commonly lasts between three and seven days.

Both estrogen and progesterone sit at their lowest point of the month during these days. That is worth noting because so much of what distinguishes the other three phases comes down to which of those two hormones is rising. The menstrual phase is the only stretch where neither is.

There is a common misreading here that the menstrual phase is separate from the follicular phase. It is not. The follicular phase begins on day one as well, which means bleeding and follicle recruitment overlap at the start of the month. The two names describe different organs, not different weeks.

What is shed, how much, and over how many days differs meaningfully between individuals and across a lifetime, which is a subject worth reading on its own terms rather than compressed into a paragraph here.

III

The follicular phase, the longest and the most variable.

The follicular phase runs from day one to ovulation. It takes its name from the ovarian follicles, each a fluid-filled sac holding an immature egg. Follicle-stimulating hormone from the pituitary recruits a cohort of them at the start of the cycle, and over the following days one becomes dominant while the others regress.

As that dominant follicle grows it produces estradiol, and estradiol climbs steadily through the second half of the phase. In the uterus, the same rise drives the endometrium to thicken again, which is why this stretch is also called the proliferative phase in the uterine description.

This is the phase that accounts for most of the difference in cycle length between one person and another, and between one month and the next in the same person. A 24-day cycle and a 34-day cycle usually differ in the follicular phase rather than the luteal one. Anything that shifts the timing of ovulation shifts the length of this phase with it.

Because it is the least uniform part of the month, it is also the part most often misdescribed in day-by-day charts, which tend to assume a textbook 28-day cycle that a minority of cycles actually match.

IV

Ovulation, the hinge of the month.

When estradiol from the dominant follicle reaches a high enough level for long enough, the pituitary responds with a sharp release of luteinising hormone. That release is the LH surge, and it is the signal that triggers ovulation roughly a day to a day and a half later. The follicle ruptures and releases its egg.

Ovulation itself is a single event rather than a stretch of days, which is why some descriptions of the ovulation cycle count three phases rather than four and fold it into the end of the follicular phase. Both conventions are in use. The four-phase version keeps it separate because it is the point the whole month turns on.

The fertile window described in fertility research spans several days, since sperm can survive in the reproductive tract for a number of days while the egg itself remains viable for under a day. The window therefore sits mostly before ovulation rather than after it.

Not every cycle includes an ovulatory event. Cycles without one are described as anovulatory, and they occur more often in the years just after menarche and in the years approaching the menopausal transition. A cycle can produce bleeding without having ovulated.

V

The luteal phase, the steadier half.

After the egg is released, the remaining follicle becomes the corpus luteum, a temporary endocrine structure that produces progesterone. That progesterone turns the thickened endometrium into a secretory lining, which is why the uterine description calls this the secretory phase.

Unlike the follicular phase, the luteal phase holds a fairly consistent length, most often between twelve and fourteen days. The corpus luteum has a limited lifespan of its own, and in the absence of a pregnancy it regresses on roughly that schedule regardless of how long the first half of the cycle ran.

As it regresses, progesterone and estradiol both fall. That fall is what withdraws support from the endometrium and begins the next period, returning the cycle to day one. Basal body temperature, which rises slightly after ovulation under the influence of progesterone, falls with it.

The late luteal days are also when premenstrual symptoms are most commonly reported, and the nutrients researchers have studied in that context are a subject of their own, examined separately rather than summarised here.

VI

How long a cycle actually lasts.

Twenty-eight days is the number almost everyone has heard, and it is a reasonable average. It is not a standard. In adults, clinicians generally describe cycles between 21 and 35 days as falling within the usual range, and a large share of cycles sit somewhere in that band rather than at its midpoint.

Variation within the same person is normal too. Cycle length commonly differs by a few days from one month to the next, and that variation sits almost entirely in the follicular phase. The luteal phase, anchored to the lifespan of the corpus luteum, moves much less.

Age changes the pattern at both ends. Cycles in the first years after menarche are often longer and less regular while the hormonal sequence establishes itself. Cycles in the years approaching the menopausal transition change again, and the nature of that change is itself one of the defining markers clinicians use to describe where someone sits on the reproductive timeline.

This is the part of the subject that connects most directly to healthy aging, since the cycle is one of the few systems in the body whose changes across the decades can be observed month by month without a laboratory.

VII

Where this fits.

This subject sits inside Systemic Balance, the fourth pillar of The Longevity Code, the pillar concerned with the body's interconnected systems rather than any single molecule. A pillar is a location within a framework. It describes a category, not an outcome for any individual reader.

The reason the cycle belongs there rather than anywhere else is that it is not one organ acting alone. It is a conversation between the brain, the ovary and the uterus, running on a monthly schedule, and readable from the outside by anyone paying attention to it. Few systems in the body offer that.

Formulations across the pillars are built around ingredients researched in the context of the systems described here. Any study referenced in this cluster was conducted independently and did not involve any specific Codeage product.