What does “fertile” mean on a period tracker?

On a period tracker, the fertile window is a group of days around your estimated ovulation. It's when you're most likely to get pregnant if you have unprotected sex. The tracker works it out from the calendar. It doesn't measure anything in your body.

How the window is worked out

Period Tracker Free estimates ovulation as 14 days before your next expected period. The fertile window runs from 5 days before that to 1 day after, so 7 days in total. The days before count because sperm can survive for several days. The day after allows for an egg that has only just been released.

With a 28-day cycle, that's days 10 to 16. With a 35-day cycle it moves to days 17 to 23. So the window is only as good as the cycle length behind it.

Why the other days aren't safe

The window is an estimate, so it can be in the wrong place. Ovulation can come earlier or later than the calendar expects, even if your periods are usually regular. Days outside the window have a lower estimated chance of pregnancy, not zero chance. That's why this tracker never labels any day as safe.

Don't use a period tracker as your only contraception. If you want to use fertility awareness, learn it from a trained teacher or clinician. ACOG and the NHS both say it needs training and daily tracking of body signs to work well.

If you're trying to get pregnant

The window gives you a rough idea of your most fertile days. The NHS suggests having sex every 2 to 3 days through your cycle instead, which is simpler and doesn't depend on an estimate being right. If you've been trying for a year without success, see a GP. Go sooner if you're 36 or over or know of a possible fertility problem.