The menstrual cycle, phase by phase

A great deal of confident content exists about cycle phases, some of it grounded in endocrinology and some of it invented, and the two are frequently presented together.

The mechanics

A cycle is counted from the first day of bleeding.

In the follicular phase, follicle stimulating hormone recruits follicles, one becomes dominant, and rising oestradiol thickens the endometrium.

Rising oestradiol eventually triggers a surge of luteinising hormone, which causes ovulation roughly a day and a half later.

The remnant follicle becomes the corpus luteum and produces progesterone, which stabilises the endometrium and raises basal body temperature slightly.

Without pregnancy the corpus luteum regresses, hormone levels fall, and the endometrium is shed.

The luteal phase is relatively fixed at around twelve to fourteen days; the follicular phase is what varies, which is why cycle length varies and why calendar prediction of ovulation is unreliable.

What normal looks like

Cycle length of roughly twenty-one to thirty-five days in adults, with some variation between cycles being ordinary.

Bleeding for up to about eight days.

Volume that is difficult to assess objectively, which is why the practical definition of heavy bleeding is that which interferes with quality of life — a definition adopted deliberately in guidelines to stop women being told their bleeding is not heavy enough to matter.

Cycles are commonly irregular in the years after menarche and in perimenopause, which is expected rather than abnormal.

Symptoms and what causes them

Period pain results from prostaglandins causing uterine contraction and reduced blood flow, which is why non-steroidal anti-inflammatory drugs work well — they inhibit prostaglandin production, and taking them before pain is established works better than after.

Premenstrual symptoms occur in the luteal phase and resolve with bleeding, and are thought to reflect sensitivity to normal hormonal fluctuation rather than abnormal hormone levels.

Premenstrual dysphoric disorder is a severe form with marked mood symptoms causing significant impairment, recognised as a distinct diagnosis, and it responds to specific treatments including SSRIs given continuously or in the luteal phase only.

Mid-cycle pain at ovulation is common and benign.

Cyclical breast tenderness, bloating, headache and bowel changes are all explicable by hormonal effects on other systems.

What warrants investigation

Bleeding between periods or after sex.

Bleeding that soaks through protection hourly, contains large clots, or requires double protection.

Pain that stops normal activity, is not controlled by simple analgesia, or is worsening.

Cycles shorter than twenty-one or longer than thirty-five days consistently in an adult, or absent periods.

Any bleeding after menopause.

And a sudden change from an established pattern.

Common findings include fibroids, polyps, endometriosis, adenomyosis, polycystic ovary syndrome, thyroid disease and bleeding disorders — the last being underdiagnosed in women with heavy periods since adolescence.

Tracking

Useful, with caveats.

Recording dates, flow, pain and symptoms produces information that is genuinely helpful to a clinician and that memory does not preserve accurately.

Apps that predict ovulation from cycle length alone are unreliable, because ovulation timing varies within individuals more than people expect.

Methods with better accuracy combine basal body temperature, which confirms ovulation after it has happened, and cervical mucus observation, which anticipates it.

Ovulation predictor kits detect the luteinising hormone surge and indicate that ovulation is likely imminent.

Data privacy is a genuine consideration with cycle apps in some jurisdictions and is worth thinking about before entering years of reproductive data.

Cycle syncing

Where the evidence needs stating plainly.

The popular framework prescribing specific foods, exercise types and work activities for each phase is not supported by good evidence, and the four-phase system with its specific recommendations is largely a commercial construction.

What is supported: symptoms vary across the cycle for many people, and adjusting activity around your own symptoms is entirely reasonable.

Research on athletic performance across the cycle has produced inconsistent results with generally small effects and high individual variation, which argues against prescriptive rules and in favour of personal tracking.

The distinction matters because the prescriptive version tells women there is a right way to live each week, which is both unfounded and one more thing to fail at.

Contraception and the cycle

Worth clarifying since it causes confusion.

The bleed on combined hormonal contraception is a withdrawal bleed rather than a period, and is not medically necessary — continuous or extended regimens are safe and are recommended in several guidelines.

The original monthly break was a design decision made for acceptability rather than for health.

Which means anyone whose life is disrupted by that bleed can generally discuss skipping it.

General information only, not medical advice. Consult a qualified clinician about heavy, painful, irregular or absent periods.