Ovulation · 6 min
The fertile window, explained
Sperm can wait. An egg does not. That timing is why a “window” exists — and why it is only an estimate.
Not contraception
Cycle tracking alone is not a reliable way to prevent pregnancy. It is not a method of contraception. If you need pregnancy prevention, talk with a clinician about methods designed for that purpose.
The basic biology
An egg lives about 12 to 24 hours after ovulation. Sperm may survive several days in the reproductive tract. The combination creates a fertile window that often spans about six days ending on or just after ovulation.
Why apps estimate
Most cycle apps, including LunaLoop, estimate ovulation by counting backward from the next expected period (a luteal phase of about 14 days is a common assumption) and by learning your personal averages. Stress, illness, travel, and thyroid or PCOS-related patterns can move ovulation. Estimates are not lab results.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby. New England Journal of Medicine, 1995.
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ, 2000.
- Office on Women's Health (U.S. Department of Health and Human Services). Your menstrual cycle. womenshealth.gov, 2025.
- American College of Obstetricians and Gynecologists. Fertility Awareness-Based Methods of Family Planning (FAQ024). ACOG, 2019.
Educational information for general awareness. This article has not been individually reviewed by a clinician for you, and it is not medical advice.