How Ovulation Works
Ovulation is the release of an egg from an ovary, roughly once per cycle. Everything this site calculates follows from one structural fact: a cycle is built from two phases with very different behaviour — and only one of them is doing the varying.
The two phases
Follicular phase (period → ovulation). From the first day of bleeding, rising FSH recruits a batch of follicles in the ovaries; one becomes dominant and matures an egg while oestrogen rebuilds the uterine lining and turns cervical mucus clear and stretchy. This phase is the flexible one: it lasts about 14 days in a textbook 28-day cycle, about 12 in a 26-day cycle and about 21 in a 35-day cycle. Stress, illness, travel and energy balance all act here — which is why a "late period" is usually a late ovulation, not a long second half.
Ovulation. A surge of luteinising hormone (LH) triggers the dominant follicle to release its egg, roughly 24–36 hours after the surge begins. The egg is fertilisable for about 12–24 hours. The five fertile days before it exist because sperm, in fertile mucus, can wait that long.
Luteal phase (ovulation → next period). The emptied follicle becomes the corpus luteum and secretes progesterone, which holds the uterine lining ready and nudges body temperature up slightly. This phase is comparatively constant — typically 12–14 days, and the reason calculators assume 14 — because it is governed by the corpus luteum's lifespan rather than by recruitment timing. If no implantation occurs, progesterone falls and the lining is shed: the next period.
Pinning the day down in this cycle, not the average one
Ovulation predictor kits (OPKs)
Urine test sticks that detect the LH surge. A positive result means ovulation is likely within the next 24–36 hours — the most useful forward-looking signal a home test gives. They are a prediction, not a confirmation: a surge occasionally occurs without an egg being released, and conditions such as PCOS can produce misleading readings.
Basal body temperature (BBT)
Progesterone raises resting temperature by roughly 0.2–0.5 °C (0.4–1.0 °F) after ovulation, and it stays up until the next period. Measuring first thing each morning and charting the shift confirms — retrospectively — that ovulation happened and on about which day. Because the rise comes after the egg is released, BBT cannot time the current cycle's window in advance; its value is pattern-building across several cycles, and it is how you would measure your own luteal phase length to feed back into the calculator on this site.
Cervical mucus and cycle signs
Mucus that turns clear, slippery and stretchy ("egg white") in the days before ovulation is a genuine forward signal, driven by the same oestrogen rise that precedes the LH surge. Some people also notice one-sided pelvic twinges (mittelschmerz) around ovulation. These signs are free and immediate, but subjective.
Why the calendar alone is not contraception
Calendar methods assume your next cycle will behave like your average. In reality ovulation day moves by several days between cycles — published typical-use failure rates for fertility-awareness and calendar methods run to roughly 2 in 10 users pregnant within a year, far higher than hormonal or barrier methods. Use this site's estimates for conception planning and cycle literacy, never as a reason to skip actual contraception. Our ovulation calculator shows the window for your dates; a clinician can help if your cycles are persistently shorter than 21 days, longer than 35, or absent.