Cycle phase decoder: which phase are you really in?

Tell it where you are in life and two numbers, and it will tell you which phase you are most likely in, what your hormones are doing, which symptoms follow from that — and why, at the level of mechanism — and what would not be expected and is worth asking a clinician about. It estimates ovulation as your cycle length minus 14, not day 14, which is the single thing most cycle tools get wrong. And in perimenopause it does what trackers refuse to do: it tells you the prediction is unreliable, and explains exactly why.

Do NOT use this to avoid or to achieve a pregnancy. It is not contraception and it is not a fertility tool. Calendar-based prediction fails often in ordinary use — ovulation moves with illness, stress, travel and sleep, and the day it lands cannot be known in advance. In perimenopause it is unreliable while pregnancy is still possible. This tool explains what your body is probably doing and why. It cannot tell you what is safe, and it will never diagnose you or tell you to start, stop or change a medicine or a contraceptive.

1. Where are you in life? Start here — for two of these answers, phase prediction does not apply at all.

My cycles are regularYour period arrives roughly when you expect it, month to month.My cycles are becoming irregular (perimenopause)Cycles are shifting — closer together, further apart, heavier, lighter, skipped.I'm on hormonal contraceptionCombined pill, patch, ring, implant, injection, or a hormonal coil.Postmenopause — no period for 12+ monthsYour last period was more than a year ago.

Pick your stage to go on. The same two numbers mean completely different things depending on the answer. On a regular cycle they estimate a phase. In perimenopause they estimate a phase that may not exist this month. On hormonal contraception and after menopause there is no follicular or luteal phase to be in, and this tool will say so rather than manufacture an answer.

Nothing you enter is stored or sent anywhere. It all stays in your browser.

See a doctor about any of these — whatever phase the tool says you are in

Bleeding between periods
Bleeding after sex
Cycles shorter than 21 days or longer than 35 days that are new for you
Periods lasting more than 7–8 days
Flooding — soaking through a pad or tampon in an hour — or clots bigger than a 10p or a quarter
ANY bleeding after 12 months with no period — this is never normal and always needs evaluating, even if it was once, even if it has already stopped
Period pain that stops you working or sleeping despite anti-inflammatories
Premenstrual symptoms that do not lift when bleeding starts, or that include suicidal thoughts

Explanations drawn from ACOG, the NHS, the Office on Women's Health, NICHD (NIH), MedlinePlus (NIH) and The Menopause Society. This tool never diagnoses, never recommends a treatment, and will never tell you to start, stop or change a medication or a contraceptive. Everything is computed in your browser — nothing you enter is stored or sent anywhere.

Why “day 14” is wrong for most women

The 28-day cycle with ovulation on day 14 is an average, and it was never meant to be a rule. Cycles between 21 and 35 days are all considered normal, and most women are not 28. So the useful question is not which day is day 14 — it is which half of the cycle actually moves.

The answer is the first half. After ovulation, the emptied follicle becomes the corpus luteum, a temporary gland with a roughly fixed lifespan of about 14 days. Unless a pregnancy implants and rescues it, it dies on schedule, progesterone collapses, and the period starts. That gives the luteal phase a near-constant length. What varies is the follicular phase— the stretch from day 1 until an egg is released. Follicle recruitment gets delayed by illness, stress, undereating, hard training, disrupted sleep and travel. A cycle that comes “late” almost always had a late ovulation, not a long second half.

Which means the arithmetic runs backwards from the period, not forwards from day 1:

Estimated ovulation ≈ cycle length − 14.

On a 32-day cycle that is around day 18. On a 24-day cycle it is around day 10. Anchoring on day 14 would put a 32-day cycle's ovulation four days early and a 24-day cycle's four days late — which is not a rounding error, it is a different phase.

The fertile window ends at ovulation. It does not start there.

The released egg is viable for about 12 to 24 hours. Sperm can survive up to five days in fertile cervical mucus. Put those two facts together and the window is roughly six days long, and it sits before ovulation and closes on the day itself. Sex four days before ovulation can lead to a pregnancy; sex two days after it usually cannot. This is the most consistently misunderstood fact in the entire cycle.

It also has a consequence people find surprising: on a short cycle, the fertile window overlaps the period. With a 22-day cycle, ovulation lands around day 8 and the window opens around day 3 — while you may still be bleeding. So no, you cannot rely on “you can't get pregnant on your period.”

None of which makes this a contraceptive. Calendar methods fail often in real use because the thing they depend on — the day ovulation lands — is the very thing that moves. Never use this tool, or any calendar, to avoid or to achieve a pregnancy.

What perimenopause does to all of this

Everything above assumes you ovulate. In perimenopause that assumption quietly breaks, and the whole model goes with it.

Ovulation becomes erratic: some cycles release an egg, some do not, and you cannot tell which from the outside. In a cycle with no ovulation there is no corpus luteum — and the corpus luteum is the only source of progesterone. So in that month there is no progesterone at all. No secretory lining, no temperature rise, no luteal phase in any meaningful sense, while estrogen carries on building the lining unopposed. That is the mechanism behind the heavy, chaotic bleeds of the forties: a lining that grew with nothing to shed it on schedule, and then went all at once. Read irregular periods in perimenopause for the full picture.

There is one early pattern worth knowing, because it is so often missed: cycles getting shorter. “My periods come every three weeks now” is one of the first signs of perimenopause — as FSH rises, follicles are recruited sooner and the follicular phase compresses. Women expect perimenopause to announce itself with skipped periods, so periods arriving closer together gets read as nothing at all.

A period-tracker app will happily keep drawing a tidy wheel through all of this, predicting an ovulation that may never happen and a luteal phase that does not exist this month. That is not a harmless simplification. It tells women their bodies are behaving predictably at exactly the point they have stopped, and it is why we show the estimate in perimenopause but label it plainly as a rough guide that can be wrong.

Why the luteal phase feels the way it does

“Hormones fluctuate” explains nothing. Here is the actual chain.

Progesterone from the corpus luteum is metabolised into allopregnanolone, a neurosteroid that binds GABA-A receptors — the same receptor family alcohol and benzodiazepines act on — and calms the brain. Its level climbs through the luteal phase and then falls sharply in the last few days as the corpus luteum fails. That fall is a withdrawal from a sedative your own body was making, and it has the shape of any GABA-ergic withdrawal: anxiety, irritability, a shortened fuse, broken sleep. The same progesterone lifts your core temperature set-point by 0.3–0.5°C, which is both why basal body temperature rises after ovulation and why the second half of the cycle sleeps worse — falling asleep depends on core temperature dropping, and the floor has been raised.

The important part: research points to PMS and PMDD being driven not by abnormal hormone levels — those are usually normal — but by differing sensitivityto that normal shift. It is a receptor-level difference, not a character flaw. Which is exactly why “just relax” has never worked for anybody.

And then the bleed itself: falling progesterone destabilises the lining, which releases prostaglandins that clamp the arteries and contract the muscle of the womb to expel it. Those prostaglandins do not stay put — they act on gut smooth muscle and inflammatory pathways too, which is the real mechanism behind period diarrhoea, nausea, headache and the flattened, fluey feeling of day one. We wrote it all up in period flu.

The line between a phase and a symptom

The most useful thing this tool does after the arithmetic is tell you what is not part of the phase. Cramps on day 2 are physiology. Bleeding on day 19 is not. A twinge at ovulation is physiology; a bleed at ovulation is not. Irritability in the last three days is physiology; irritability that does not lift when you bleed is something else, and PMS is the wrong frame for it.

Whatever the tool says, take these to a clinician: bleeding between periods, bleeding after sex, cycles newly shorter than 21 or longer than 35 days, periods lasting more than 7–8 days, flooding or large clots — and any bleeding at all after 12 months without a period, which is never normal and always investigated. Start with postmenopausal bleeding and the bleeding decoder, which reads the same bleeding pattern differently depending on which stage you are in.