What Is the Fertile Window?
The fertile window is the span of days each cycle when pregnancy is biologically possible. It lasts approximately 6 days: the 5 days before ovulation and the day of ovulation itself.
This window exists because of a timing mismatch: sperm can survive in the female reproductive tract for up to 5 days, while a released egg is viable for only 12–24 hours. So the sperm need to already be waiting when the egg arrives.
When Does the Fertile Window Happen?
In a textbook 28-day cycle, ovulation occurs around day 14, making the fertile window roughly days 9–14. But here's what the research actually shows:
A landmark study by Wilcox et al. published in the BMJ (2000) found that ovulation timing varies widely—even among women with regular cycles. Only about 30% of women have their fertile window entirely within the days identified by standard clinical guidelines.
This means estimating your fertile window based on cycle day alone can be unreliable. That's why tracking additional signs of ovulation is so valuable.
How to Identify Your Fertile Window
Ovulation Predictor Kits (OPKs)
OPKs detect the surge of luteinizing hormone (LH) that occurs 24–36 hours before ovulation. A positive OPK tells you ovulation is likely imminent, giving you a clear signal to time intercourse.
Recommended: Ovulation Test Strips
Affordable OPK strips give you the same accuracy as expensive digital tests. Start testing a few days before you expect ovulation based on your cycle length.
Browse OPK Test Strips on Amazon →Basal Body Temperature (BBT)
Your resting body temperature rises by about 0.5–1.0°F after ovulation due to progesterone. While BBT tracking confirms ovulation happened, it can't predict it in advance—the temperature rise occurs after ovulation.
BBT is most useful for confirming your cycle patterns over several months, helping you predict future fertile windows.
Cervical Mucus Changes
As you approach ovulation, cervical mucus becomes clear, stretchy, and slippery—often described as resembling raw egg whites. This "fertile-quality" mucus helps sperm survive and travel to the egg. Monitoring mucus changes is free and can be a reliable indicator of fertility.
The Peak Fertility Days
Not all days within the fertile window are equally fertile. Research consistently shows the highest probability of conception on the 1–2 days immediately before ovulation. Conception rates drop sharply on ovulation day itself and are essentially zero the day after.
Key Takeaway
Don't wait for ovulation to have sex. The best strategy is to have intercourse in the days leading up to ovulation, ideally every 1–2 days during your estimated fertile window.
Use our ovulation calculator to estimate your fertile window based on your cycle length.
This guide was last reviewed on January 18, 2026.