Understanding Ovulation & The Fertile Window
The science behind the LH surge, egg release, and the biological window of fertility.
Cycle literacy - Fertility & Ovulation, about 3 min read
The Ovulatory Surge
When estrogen levels reach a critical peak, they trigger a rapid surge of Luteinizing Hormone (LH) from the pituitary. Usually about 24 to 36 hours after this surge begins (sometimes up to about two days), the dominant follicle ruptures, releasing a mature egg.
Once released, the egg survives in the fallopian tube for only 12 to 24 hours. If it is not fertilized within this timeframe, it naturally dissolves.
The Fertile Window
Although the egg lives for only about a day, sperm can survive inside the body for several days — usually up to 5, and the NHS notes up to 7 days in the fallopian tubes. That's why pregnancy is most likely from sex in the 5 days before ovulation, on the day of ovulation, and — by ACOG's count — the day after, and why sex a little earlier isn't completely without risk.
Important note: the dates in this app are estimates from past cycle lengths. They are not a birth control method and must not be used to prevent pregnancy.
Sources
- American College of Obstetricians and Gynecologists: Fertility Awareness-Based Methods of Family Planning
- Peer-reviewed literature: Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby (N Engl J Med 1995;333:1517-21)
- National Health Service (UK): Periods and fertility in the menstrual cycle