Why the window opens before ovulation
Sperm can survive in the reproductive tract for up to five days, while an egg is viable for roughly 24 hours after release. The fertile window therefore starts about five days before ovulation and closes a day after it.
Ovulation is timed backwards from the next period, not forwards from the last one. The luteal phase is relatively fixed at 12–16 days, while the follicular phase before ovulation is what varies between people and between cycles.
Signs of ovulation beyond the calendar
Cervical mucus changes predictably through the cycle, becoming clear, stretchy and slippery around ovulation — often compared to raw egg white. This is one of the more reliable everyday signs and typically appears in the days just before the egg is released.
Basal body temperature rises by roughly 0.3°C after ovulation and stays elevated. Because the rise comes after the event, temperature charting confirms ovulation retrospectively rather than predicting it, which makes it useful for learning your pattern over several cycles.
Ovulation predictor kits
These detect the surge in luteinising hormone that precedes ovulation by 24 to 36 hours, so a positive test marks the start of the most fertile window. They are considerably more reliable than calendar prediction, particularly with irregular cycles.
A positive result confirms the hormone surge, not that an egg was actually released. Where cycles are consistently irregular or tests never turn positive, that is worth discussing with a clinician.
Frequently asked questions
How accurate is this for irregular cycles? +
Considerably less so. If your cycle length varies by more than a few days, calendar prediction is unreliable — basal body temperature or ovulation predictor kits track the actual event.
Can I use this as contraception? +
No. Calendar-based prediction has a high failure rate and should never be relied on to prevent pregnancy.
Can I ovulate more than once in a cycle? +
Multiple eggs can be released, but only within roughly 24 hours of each other — which is how non-identical twins occur. A second, separate ovulation later in the same cycle does not happen.
How long should I try before seeking advice? +
General guidance is a year of regular unprotected intercourse under 35, or six months over 35. Earlier if cycles are very irregular or absent.