The fertility window is the time in your cycle when conception is possible. Most calculators show six days — biologically, ten makes more sense.
This guide explains how the expanded window, the LH surge, and the peak days fit together — and when conception probability is highest.
Classic vs. expanded fertility window
The classic window covers six days: five days before ovulation plus ovulation day itself. The logic: sperm survive up to five days in cervical mucus, while the egg is fertilizable for only 12–24 hours.
The expanded window covers ten days — from day −7 to day +2 relative to ovulation. It accounts for variability in ovulation timing and occasional long sperm survival.
Why ten days?
Research shows actual ovulation day varies by ±2 days even in "regular" cycles. A ten-day window catches every fertile day, even when ovulation drifts.
The LH surge — the most reliable marker
Luteinizing hormone (LH) spikes 24–36 hours before ovulation. This LH surge triggers ovulation itself.
Over-the-counter LH tests (also called "ovulation tests") detect the hormone in urine. A positive test means ovulation will occur within the next 1–2 days.
Tip
Start testing 3–4 days before predicted ovulation. Late morning or early afternoon urine is best — first-morning urine can distort the result.
Daily conception probability
The landmark Wilcox et al. 1995 NEJM study first quantified daily conception probabilities:
| Day | Probability | Rating |
|---|---|---|
| −5 | 10 % | Low |
| −4 | 16 % | Moderate |
| −3 | 14 % | Moderate |
| −2 | 27 % | High |
| −1 | 31 % | Very high |
| 0 (ovulation) | 33 % | Highest |
| +1 | 10 % | Low |
The peak days (day −2 through ovulation day) account for roughly 90 % of pregnancies in the data. If you are trying to conceive, this is where to focus.
How to use the fertility window
1. Track your cycle length
Track at least three months. Day 1 = first day of your period. Take the average.
2. Calculate your window
Enter your last period start and cycle length. The calculator returns a 10-day window with daily probabilities.
3. Use LH tests in the second half
From day −4 onward test daily until positive. Ovulation follows 24–36 hours later.
4. Intercourse every 1–2 days
During peak days (day −2 to day 0). Daily intercourse gives no benefit — sperm quality is best with 1–2 day spacing.
What if my cycle is irregular?
If your cycle varies by more than 7 days or falls outside 21–35 days, calendar prediction is unreliable. Better tools:
- LH tests — track the surge directly
- Basal body temperature — confirms ovulation retrospectively (0.2–0.5 °C rise)
- Cervical mucus — clear and stretchy at peak fertility
- Fertility monitors — combine LH + estradiol detection
For PCOS, very irregular cycles, or absent ovulation, see a gynecologist.
Calculate your fertility window
10-day window with LH surge and daily probability — free.
Calculate now →Related calculators
Bottom line
Your fertility window is more than six days — ten makes biological sense. Combine calendar tracking with LH tests to hit the peak days reliably.
Note: no calculator replaces medical advice. If you have been trying for 12 months (or 6 months if over 35) without success, see a reproductive specialist.
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