The cycle in brief
A menstrual cycle is counted from the first day of one period to the first day of the next. The average is about 28 days, but anywhere from 21 to 35 days is normal. It runs in four phases driven by shifting hormones.
The four phases
1. Menstrual phase (days ~1–5)
Your period. The uterine lining sheds, hormone levels are low, and energy is often lower. Rest and gentle movement can feel good.
2. Follicular phase (days ~1–13)
Overlapping with your period and continuing after, oestrogen rises as the body prepares to release an egg. Many people feel more energetic, focused, and upbeat as this phase progresses.
3. Ovulation (around day ~14)
An egg is released — usually about 14 days before your next period. This is the most fertile time. Some notice a small temperature rise or changes in cervical mucus. Our ovulation calculator estimates this window.
4. Luteal phase (days ~15–28)
Progesterone rises. If pregnancy doesn't occur, hormones fall toward the end, which can bring PMS symptoms — mood changes, bloating, fatigue, cravings — before the next period begins.
Why tracking helps
Tracking your cycle reveals your personal patterns — average length, symptoms, energy shifts — so nothing catches you off guard. It helps you plan, spot irregularities early, and understand your body. Use our period calculator to predict upcoming dates.
What's normal — and when to check in
Some month-to-month variation is normal. See a healthcare professional if your cycles are very irregular, suddenly change, are extremely heavy or painful, or stop unexpectedly — and bring your tracked data, it's genuinely useful information.
Why one phase varies and the other does not
This single fact explains most of what is confusing about cycle prediction, and it is rarely stated plainly.
The luteal phase — from ovulation to your next period — is governed by the lifespan of the corpus luteum, the structure left behind after the egg is released. That lifespan is relatively fixed, usually 12 to 14 days, and reasonably consistent within an individual.
The follicular phase — from the first day of your period to ovulation — has no such constraint. It continues until a follicle matures sufficiently to trigger ovulation, and that process is sensitive to stress, illness, travel, sleep disruption, significant weight change, and large increases in training load.
So when a cycle runs 32 days instead of 28, it is almost always because the follicular phase was longer, not because the luteal phase stretched. This is why ovulation predictions count backwards from your expected next period rather than forwards from your last one — and why calendar-based prediction becomes unreliable precisely when your cycles are irregular, since irregularity lives in the unpredictable phase.
Tracking, and what is worth recording
The value of tracking is not the prediction. It is building a record of your own normal, because population averages tell you very little about any individual.
Dates alone are a reasonable start, but a few extra fields make the record considerably more useful: flow heaviness, pain and where it sits, mood, energy, sleep quality, and any recurring symptoms. Patterns invisible month to month become obvious across six.
This also has real clinical value. If you ever need to discuss your cycle with a doctor, arriving with several months of data changes the conversation entirely — it replaces "I think they're irregular sometimes" with a record they can actually assess. Conditions such as PCOS, endometriosis, fibroids and thyroid dysfunction are common and frequently diagnosed years later than they could have been, partly because symptoms get normalised and partly because nobody has the data.
Worth flagging to a professional: cycles consistently shorter than 21 or longer than 35 days, a sudden change from your established pattern, three or more missed periods without pregnancy, bleeding heavy enough to soak through protection hourly, periods lasting more than seven days, pain that interferes with daily life, or bleeding between periods. Period pain severe enough to stop you functioning is not something to endure quietly.
Training and eating across the cycle
Interest in adjusting training and nutrition to cycle phases has grown considerably, and the honest summary is that the evidence is still thin and inconsistent. Some people notice meaningful differences in strength, energy and recovery across their cycle; others notice very little, and studies attempting to find consistent performance patterns have produced mixed results.
What is reasonable, given that, is to track your own experience rather than adopting a prescriptive phase-based programme. If you consistently feel stronger in the follicular phase and flatter in the late luteal phase, planning harder sessions accordingly is sensible. If you notice no pattern, there is no evidence you should manufacture one.
Symptoms severe enough to prevent training or working are a different matter, and worth raising with a doctor rather than working around.