The basics
When is the fertile window?
The fertile window is the stretch of days before and on the day of ovulation when sex can lead to pregnancy. It moves from cycle to cycle, even for people with regular periods, so any fertile days an app shows are an estimate, not a measurement. It is not a reliable way to avoid pregnancy.
Why is the fertile window hard to predict?
Research that followed women with daily hormone tests found the fertile window was hard to predict, even for people with regular cycles. Only about 30% had their whole window between days 10 and 17, the days most often quoted.
So when Seventh shows likely fertile days, it’s an estimate from your past cycles, not a measurement.
Can I use the fertile window as birth control?
Counting days is not a reliable way to avoid pregnancy. Even fertility-awareness methods that use temperature and other signs have a typical-use failure rate of 2 to 23 in 100 people a year, according to the CDC; the NHS puts natural family planning at about 76% effective with typical use. If you don’t want to get pregnant, use contraception.
Frequently asked questions
How long can sperm and an egg survive?
An egg survives about 24 hours after ovulation. Sperm can survive inside the body for about 3 days, sometimes up to 5. That is why pregnancy is possible from sex up to 5 days before ovulation, as ACOG explains.
Is the fertile window always days 10 to 17?
No. Research that followed women with daily hormone tests found only about 30% had their whole window between days 10 and 17.
Can I get pregnant just after my period?
It’s less likely, but it can happen, especially with short cycles. See can you get pregnant on your period.
Sources
Seventh is a free period and cycle tracker for iPhone and iPad, by doogud. This article is general information, not medical advice. If something feels wrong, see a doctor.
