What is the fertile window?
The fertile window is the approximately 6-day period each menstrual cycle during which unprotected sex can result in pregnancy. It is defined by two biological facts:
- Egg viability: After ovulation, the released egg remains viable for just 12–24 hours.
- Sperm survival: Sperm can survive in the female reproductive tract for 3–5 days in the right cervical mucus environment.
This means sex on any of the 5 days before ovulation can result in pregnancy (because sperm can survive until ovulation occurs), as can sex on the ovulation day itself.
Conception probability by cycle day
A landmark study by Wilcox et al. (1995) in the New England Journal of Medicine, based on 221 cycles from 221 women, measured daily pregnancy probability across the fertile window. The highest probabilities were on:
- 2 days before ovulation: ~27–33%
- 1 day before ovulation: ~31–33%
- Ovulation day: ~25–30%
- 3 days before ovulation: ~15%
- 4 days before ovulation: ~10%
- 5 days before ovulation: ~3–5%
These per-cycle probabilities apply to couples with normal fertility. The cumulative probability over several months is much higher — for healthy couples under 35, approximately 85% conceive within 12 months of trying.
How to identify your fertile window
The fertile window's timing shifts with your cycle length. For a 28-day cycle, it falls roughly around days 9–14. For a 35-day cycle, it's around days 16–21. Our ovulation calculator estimates your fertile window from your last period date and cycle length.
For greater accuracy, combine calendar calculation with physical fertility signs:
- Cervical mucus: As ovulation approaches, cervical mucus becomes more abundant and increasingly clear, slippery, and stretchy — resembling raw egg white. This "egg white cervical mucus" (EWCM) is the most reliable and cost-free sign of approaching ovulation.
- Basal body temperature (BBT): After ovulation, progesterone causes a sustained temperature rise of 0.2–0.5°C. Tracking BBT daily (before getting out of bed) confirms ovulation occurred, though it can't predict ovulation in advance.
- Ovulation predictor kits (OPKs): Detect the LH surge that precedes ovulation by 24–36 hours. The most accurate method for predicting the fertile window in advance.
The fertile window and contraception
While knowing your fertile window is valuable for trying to conceive, it is not a reliable method of contraception. Ovulation can vary by several days from cycle to cycle, especially under stress or illness. Calendar-based methods (rhythm method) have a typical-use failure rate of approximately 24% per year, compared to <1% for long-acting reversible contraception.