Fertile window
The roughly six days each cycle when sex can lead to pregnancy: about five days before ovulation plus the day itself.
What does fertile window mean?
Fertile window: The roughly six days each cycle when sex can lead to pregnancy: about five days before ovulation plus the day itself.
How it is used in the Splash library
In a study of 237 women trying to conceive, a wrist bracelet combining skin temperature, pulse and other signals could identify the fertile window with reasonable accuracy.
From “Basal body temperature, explained”Avoiding sex or using condoms throughout the fertile window
From “Fertility awareness methods: the honest guide”Sperm can survive for several days, which is why the fertile window starts before ovulation.
From “The four phases, in depth”That study (221 women trying for a baby, with regular cycles) also found that every day from day 6 to day 21 had at least a 10% chance of being in the fertile window.
From “Why your predictions shift”That little stretch is called your Fertile window, and once you understand it, a lot of fertility advice suddenly makes sense.
From “Your fertile window, explained”
Fertile window explained
The fertile window is the set of days in each cycle when sex can lead to pregnancy. It is about six days long: the five days before ovulation and ovulation day itself, because sperm can survive up to five days and an egg lives for about a day.
The highest chance of pregnancy is from sex in the two days before ovulation and on the day of ovulation (Wilcox, 1995).
What's typical
On a 28-day cycle the window is often around days 9 to 15, but this moves with your cycle length. In one study, the fertile window fell on different days for most people, and even with regular cycles it could start as early as day 6 or as late as day 21 or later (Wilcox, 2000).
Signs that you are in your fertile window include slippery, stretchy cervical mucus and a positive LH test.
Knowing the window is useful when trying to conceive. Calendar estimates of it are not a reliable way to avoid pregnancy.
When to talk to a GP, nurse or pharmacist
- You have been trying to conceive for a year (or 6 months if you are 36 or over) without success (NHS).
- Your cycles are very irregular and you can't tell when you might ovulate.
- You want to use fertility awareness as contraception: the NHS advises learning it from a trained teacher, using several signs.
In the UK you can call NHS 111 for advice at any time. In an emergency call 999 (UK) or 911 (US). This is general information, not medical advice.
Where it comes up
Sources
Sources checked October 2026.
See it in your own cycle.
Log a few periods and Splash shows your patterns, privately, on your phone. Free, no account.