What the cycle is
The menstrual cycle is the body's monthly preparation for a possible pregnancy, orchestrated by a rise and fall of hormones — mainly oestrogen and progesterone, conducted by signals from the brain. It is counted from the first day of one period to the day before the next. The textbook figure is 28 days, but that is an average, not a rule: anywhere from about 21 to 35 days is considered typical for adults, and your own length can shift from month to month.
The four phases
Menstrual phase (roughly days 1–5). If no pregnancy occurred, the thickened lining of the uterus is shed — this is your period. Hormone levels are at their lowest, which for many people coincides with lower energy.
Follicular phase (roughly days 1–13, overlapping the period). The brain prompts the ovaries to ripen follicles, each containing an egg. Oestrogen rises steadily, rebuilding the uterine lining. Many people report rising energy and mood through this phase as oestrogen climbs.
Ovulation (around day 14 in a 28-day cycle). A surge of luteinising hormone releases a mature egg from the ovary. The egg survives for about 12 to 24 hours. Because sperm can live for several days, the "fertile window" spans roughly the five days before ovulation plus the day itself. Crucially, ovulation timing is the most variable part of the cycle.
Luteal phase (roughly days 15–28). The emptied follicle becomes a structure that produces progesterone, which maintains the lining in case of pregnancy. If the egg is not fertilised, progesterone falls, triggering the next period. Premenstrual symptoms often appear in this phase as hormones drop.
Why your cycle varies
A "perfect" 28-day cycle with ovulation exactly on day 14 is a simplification that real bodies rarely follow precisely. Stress, illness, travel and jet lag, significant weight change, intense exercise, poor sleep, hormonal contraception, perimenopause, and conditions such as polycystic ovary syndrome (PCOS) or thyroid disorders can all lengthen, shorten, or skip cycles. This natural variability is exactly why calendar-based prediction is only ever an estimate. Our period tracker predicts your next period by adding your typical cycle length to your last start date, and stores everything only on your own device — but it is for tracking and planning, never for medical decisions.
Tracking, and what it can and can't tell you
Keeping a simple record of when your periods start teaches you your own normal. Over a few months a pattern usually emerges: roughly how long your cycle runs, how much it swings, and what your typical period feels like. That baseline is genuinely useful — it makes it easier to notice when something changes, to plan around your period, and to give a doctor accurate information if you ever need to. The most basic version is just marking the first day of bleeding on a calendar.
What tracking cannot do is make biology more predictable than it is. Because ovulation timing shifts, calendar predictions are educated guesses, not appointments. Apps and trackers that promise to pinpoint your fertile window from dates alone are estimating, and the estimate is roughest exactly where it matters most. Body-based fertility-awareness methods add signals like waking temperature and cervical mucus precisely because dates on their own are not enough — and even those require proper training to use.
Common cycle experiences
Plenty of normal variation gets mistaken for a problem. Premenstrual symptoms — bloating, breast tenderness, mood changes, food cravings — are common in the luteal phase as progesterone falls, and range from barely noticeable to genuinely disruptive. Cycle length changing with life stage is expected: cycles are often irregular in the first years after periods begin and again approaching menopause. Mild cramping during a period is usual, caused by the uterus contracting. None of these on their own signals something wrong.
What is worth attention is a clear change from your own normal, or symptoms that interfere with daily life. Periods that suddenly become much heavier, much more painful, far apart, or that stop without an obvious explanation deserve a proper look — which is the point of knowing your baseline in the first place.
An essential safety note
Calendar prediction must never be used as contraception or to plan a pregnancy. Because ovulation timing shifts so much, simple date math is not a reliable fertility-awareness method on its own and can fail. For contraception or conception, speak to a healthcare provider about methods proven and appropriate for you.
The cycle and the rest of your health
Cycles don't run in isolation — sleep, stress, and activity all interact with how you feel across the month. You may find our sleep calculator and stress check-in useful companions. And see a healthcare provider if your periods are very irregular, unusually heavy or painful, stop unexpectedly, or change suddenly, as these can be worth investigating. Nothing here is medical advice.