Ovulation calculator — fertile window

Fertile days are the period in the menstrual cycle when conception is possible. They typically cover 5 days before ovulation, ovulation day itself and 1 day after — about 7 days in total. In a standard 28-day cycle the fertile window falls roughly on days 9–15.

The ovulation day is calculated by subtracting the luteal phase length (usually 14 days) from the total cycle length. For a 28-day cycle: 28 − 14 = day 14. The calculator does this automatically once you enter your cycle length.

The luteal phase is the time from ovulation to the start of the next period. It is relatively constant at 12–16 days (14 days by default). Unlike the follicular phase, the luteal phase changes little between cycles, allowing more precise ovulation prediction.

Yes — ovulation always precedes menstruation by a fixed number of days (the luteal phase), but its timing from the start of the cycle changes with cycle length. With a 21-day cycle ovulation falls around day 7; with a 35-day cycle around day 21.

An egg is viable for fertilisation for 12–24 hours after ovulation. Intercourse is therefore most effective 1–3 days before ovulation, when sperm (surviving 3–5 days) are already present in the reproductive tract and can fertilise the egg right after it is released.

Sperm can survive in the female reproductive tract for 3 to 5 days, and in a favourable environment (ovulatory mucus) up to 5–7 days. Intercourse several days before ovulation can therefore still lead to fertilisation.

Signs of approaching ovulation include: a change in cervical mucus consistency (clear, stretchy, resembling raw egg white), a rise in basal body temperature of about 0.2–0.5°C after ovulation, mild one-sided lower abdominal pain (Mittelschmerz), increased sex drive and breast tenderness.

Yes — the calendar method is least reliable with irregular cycles. In that case it helps to track the cycle for several months and take into account the shortest and longest cycle lengths. Ovulation tests (LH tests) or monitoring basal body temperature (symptothermal method) are more reliable.

No — the calculator is for informational purposes only. The calendar method has an effectiveness of about 75–88% with typical use and does not protect against sexually transmitted infections. For reliable contraception consult a gynaecologist.

If a couple has regular intercourse (every 2–3 days) for 12 months without achieving pregnancy (or 6 months for women over 35), a visit to a gynaecologist or fertility specialist is recommended. It is also worth consulting earlier if the cycle is very irregular, shorter than 21 or longer than 35 days.