Phases of the menstrual cycle

The four phases of the menstrual cycle

The menstrual cycle is a complex biological process driven by the interaction of hormones. Understanding its stages makes changes in mood, energy and physical state easier to place.

First, the main point: the phases are not the same length in everyone. There are no fixed "day numbers" pinned to the phases below — that would be untrue for most women. Where a number does appear, it is calculated from the length of your own cycle rather than taken as a general rule.

Who actually runs the cycle

Before listing the phases it is worth naming the participants, because they come up throughout — and without them the phases look like an arbitrary division.

• The pituitary gland — a gland in the brain. It releases FSH (follicle-stimulating hormone) and LH (luteinising hormone). These are the instructions.
• The ovaries — carry out the instructions and, in response, produce oestrogen and progesterone.
• The endometrium — the inner lining of the uterus. It responds to oestrogen and progesterone: it builds up and it sheds.

The cycle is a closed loop: ovarian hormones act on the pituitary, which changes its instructions, and the ovaries respond again. That is why any external disturbance — illness, sharp weight loss, severe stress — can shift the whole chain.

1. The menstrual phase (begins on the first day of bleeding)

The first day of the period is counted as the start of the cycle. Oestrogen and progesterone fall, which causes the endometrium to shed.

• Energy: often at its lowest.
• What people usually notice: cramping low in the abdomen, wanting warmth, less stamina.
• What usually helps: unhurried rest and enough fluids.

The period itself is most often described as lasting 2–7 days. Heaviness and duration are worth recording: a sharp change compared with your own usual picture matters more than any average figure.

2. The follicular phase (from the start of the cycle to ovulation)

The pituitary releases follicle-stimulating hormone (FSH), which stimulates ovarian follicles to grow. One of them becomes dominant. Oestrogen begins to rise, rebuilding the uterine lining.

• Energy: starts to come back.
• Length: this is the phase that differs between women and between one woman's own cycles — which is why overall cycle length floats mainly because of it.
• What people usually notice: strength gradually returning, better mood, more stamina under load.

Formally the follicular phase begins at the same time as the period — so the first days of the cycle belong to both at once. That is not a flaw in the division but a consequence of the phases being named on different grounds: one for what is visible, the other for what is happening in the ovary.

3. Ovulation (roughly 14 days BEFORE the next period)

A sharp surge of luteinising hormone (LH) releases a mature egg from the follicle. This is the shortest phase, and fertility is highest around it.

Why "day 14" is a myth for most people. Ovulation is counted backwards from the next period, not as "the middle of the cycle". The stable part of the cycle is the luteal phase: around 14 days on average (usually 11–17). So:

• on a 28-day cycle ovulation does indeed fall around day 14;
• on a 32-day cycle — around day 18;
• on a 24-day cycle — around day 10.

That is how Kvit calculates it too: from the expected start of the next period, not by halving the cycle. By default it takes the same roughly 14 days, but if you measure basal body temperature, Kvit determines your own luteal phase (accepting values from 9 to 18 days) and calculates from that — in which case your day of ovulation may differ from the examples above.

• Energy: the peak of activity and libido.
• What people sometimes notice: a brief one-sided ache low in the abdomen, a change in discharge.

For the calculation in detail, and what can be measured instead of calculated, see How ovulation is actually calculated.

4. The luteal phase (from ovulation to the next period)

The follicle becomes the corpus luteum, which produces progesterone. If fertilisation has not occurred, the corpus luteum regresses, hormone levels fall, and the body prepares for a new period — which is where PMS symptoms come from.

• Energy: a gradual decline, wanting comfort.
• What people often notice: sensitivity to salt and caffeine can be higher in this phase, breasts more tender, sleep less even.
• Length: the steadiest part of the cycle — usually 11–17 days.

Progesterone is also why basal body temperature is slightly higher in this phase. That is what makes it possible to see on a chart that ovulation happened — but only after the fact.

How long a "normal" cycle is

21–35 days is the range most often described as usual for adults. But it is a range, not a measure: a 26-day cycle and a 33-day cycle are equally normal if they are yours and steady.

More important than an average figure is your own variability. A few days' difference between cycles is ordinary. A sharp, sustained shift compared with how things used to be for you is what is worth recording and showing to a clinician.

And one more thing: a cycle shorter or longer than the "classic" 28 days does not mean some phase is "wrong". As the four phases above show, almost all of the difference falls on the follicular phase — that is what stretches or shortens.

Why cycles shift

Cycle length responds to many things, and none of them is a diagnosis on its own: stress, a noticeable change in weight, a sharp change in training load, illness, crossing time zones and disrupted sleep, some medicines, breastfeeding, approaching menopause.

On what to do about a single late period, and when it stops being a single one, see A late period: what it means.

What is worth recording

A single figure says nothing. A picture across several cycles says a lot:

• the first and last day of the period;
• how heavy it was;
• symptoms and the days they fell on;
• mood.

That is the practical reason to keep records: not so the app can hand you a diagnosis, but so that you have your own dates when they are needed.

What the phases do not explain

One last thing, and worth saying plainly, because a great deal of surplus has been built around phases.

The phase of your cycle is not an explanation for everything happening to you. A bad mood in the luteal phase might be PMS, or it might be tiredness, poor sleep or a bad week. The temptation to explain anything at all by hormones is strong precisely because the explanation is always at hand.

Records are useful because they let you tell the difference: a symptom that recurs in the same phase cycle after cycle is a pattern. A symptom that happened once is just a day.

When to see a clinician

• no period for three months or more, with pregnancy ruled out;
• periods stopped before the age of 45;
• bleeding between periods;
• very heavy bleeding or severe pain;
• a cycle that has suddenly become very irregular.