The menstrual cycle, in plain language.

Most explanations of the menstrual cycle are either a clinical diagram or a wellness sales pitch. This is neither. It’s the plain version: what each part of the cycle is, roughly when it happens, and why your body almost never follows the tidy 28-day chart.

Each section below is a short tour; every one links to a full, calm explainer if you want to go deeper. None of this is medical advice — it’s the context that makes your own tracking make sense.

The cycle is four parts, not 28 identical days

A cycle runs from the first day of one period to the first day of the next, and it moves through four parts:

• Menstruation — your period. The uterine lining sheds. Typically 3–7 days.
• The follicular phase — the lining rebuilds and a follicle matures. This is the part that varies most in length, which is the real reason cycles aren’t identical month to month.
• Ovulation — a single egg is released. It’s a moment, not a phase, and everything else is timed around it. More in what is ovulation?
• The luteal phase — the ~12–14 days after ovulation, when energy often dips and PMS can show up. Our full explainer: what is the luteal phase?

The 28-day cycle is an average, not a rule

Textbooks say 28 days because it’s a convenient midpoint. Real cycles run anywhere from about 21 to 35 days, and the same person’s cycle often varies by several days from one month to the next. That variability is normal — and it’s precisely why no app can promise an exact prediction. We dig into the science in why your period tracker keeps getting the prediction wrong.

Ovulation and the fertile window

Ovulation is the hinge of the whole cycle. The fertile window — the days conception is possible — is roughly the five days before ovulation plus the day itself, because sperm can survive while the egg lasts about a day. Because ovulation timing shifts, the window moves too, which is why calendar math is a rough guide rather than a precise one. The full picture is in what is the fertile window?

When your period is late

A late period sets off alarm bells, but it’s usually one of the ordinary culprits: stress, travel, sleep disruption, a change in exercise or weight, or a passing illness. Conditions like thyroid imbalance and PCOS can also shift timing. We list the common reasons — and the point at which it’s worth a doctor — in period late but not pregnant? 8 common reasons.

What tracking is actually good for

Tracking won’t give you a crystal ball. What it gives you is a baseline — your own normal — so that when something shifts, you notice early and have a clear record to bring to a clinician. That’s the calm, honest use of a tracker, and it’s the only thing we built Dew to do: hold your pattern, privately, without turning your cycle into a product.

If privacy is what brought you here, start with the private period tracker guide.