Ovulation — the release of an egg from the ovary — lasts approximately 12–24 hours. The egg survives for up to 24 hours after release. But your fertile window — the days when intercourse can result in pregnancy — spans 5–6 days. Understanding why these numbers differ is key for both trying to conceive and natural family planning.
Ovulation is triggered by a surge in luteinizing hormone (LH), detected by OPK strips. Approximately 24–36 hours after the LH surge, the dominant follicle ruptures and releases a mature egg. The egg then travels through the fallopian tube toward the uterus. During this transit — lasting 12–24 hours — the egg can be fertilized by sperm.
Sperm can survive in the female reproductive tract for up to 5 days under optimal conditions (cervical mucus plays a key role — egg-white mucus protects and nourishes sperm). This means that intercourse 5 days before ovulation can still result in fertilization when the egg is released. The fertile window therefore spans approximately:
The day after ovulation, the egg has typically already passed or disintegrated — pregnancy from intercourse on this day is unlikely.
Detect the LH surge 24–36 hours before ovulation. A positive OPK means: begin timing now. The fertile window is currently open or about to open.
The temperature rise (0.2°C+) confirms ovulation happened. Since this is after ovulation, it's useful for confirming timing but not for predicting in real time in the current cycle — over multiple cycles, it shows your consistent ovulation timing.
Egg-white cervical mucus is the real-time indicator of peak fertility. Its presence signals the optimal fertile window without any test strips. When mucus is egg-white, you are in or approaching your most fertile days.
Apps like WomensPal combine your cycle history, BBT, OPK, and cervical mucus data to estimate your fertile window based on your actual patterns — not a generic 28-day formula.
Optimal TTC timing: Aim for intercourse every 1–2 days during the 5 days before expected ovulation through the day of ovulation. You do not need to time it to the exact hour of ovulation — sperm viable in the tract when the egg arrives is what matters.
About 20% of people experience mittelschmerz — a localized pain or aching on one side of the lower abdomen during ovulation, typically lasting a few minutes to a few hours. Some also notice increased libido, breast tenderness, or mild bloating at ovulation. These signs are useful corroborating indicators but not reliable enough to use alone for timing.
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