Understanding your cycle: how period tracking works.

Phase 1 — Menstruation (Days 1–5): The uterine lining sheds. Estrogen and progesterone are at their lowest. Energy tends to be lower; rest is important.

Phase 2 — Follicular Phase (Days 1–13): Overlaps with menstruation. Follicle-stimulating hormone (FSH) triggers follicle growth in the ovaries. Estrogen rises, energy and mood often improve.

Phase 3 — Ovulation (Day 14, approximately): A surge in luteinizing hormone (LH) triggers the release of an egg. This is your most fertile window. Many people notice increased energy, libido, and social confidence.

Phase 4 — Luteal Phase (Days 15–28): Progesterone rises to prepare the uterus for potential pregnancy. If no fertilization occurs, both hormones drop and the cycle begins again. PMS symptoms often occur in this phase.

A typical cycle is 21–35 days, with menstruation lasting 2–7 days. However, "normal" varies significantly between individuals. What matters most is consistency — your personal pattern is more informative than any average.

Cycles can be affected by stress, illness, significant weight changes, travel, and hormonal contraception. Occasional irregularity is common; persistent irregularity is worth discussing with a doctor.

The fertile window is the 5–6 days around ovulation when pregnancy is possible. Sperm can survive in the reproductive tract for up to 5 days, so you can conceive from sex that occurred before ovulation.

Signs of ovulation include: a slight rise in basal body temperature (BBT), changes in cervical mucus (becomes clearer and more stretchy, like egg whites), and sometimes mild pelvic pain (mittelschmerz).

Our Period Tracker uses your cycle history to predict future periods and fertile windows. The more data you enter, the more accurate the predictions become.

We use a rolling average of your recent cycle lengths to calculate predictions, with adjustments for your typical period duration. All calculations happen in your browser — your data never leaves your device.