Understanding your fertile window

Whether you're hoping to conceive or hoping not to, the same handful of days each cycle matter most. Here's what the "fertile window" really is, how to find yours, and why it moves around.

What the fertile window is

You can only conceive on a few days each cycle. That's because an egg lives for around 24 hours after it's released, while sperm can survive in the body for up to about five days. Put those together and you get a window of roughly six days: the five days leading up to ovulation, plus ovulation day itself.

The two or three days right before ovulation are the most fertile of all, that's when sperm are already waiting as the egg arrives.

How ovulation timing works

Many of us were taught that ovulation happens on "day 14". That's only true for a textbook 28-day cycle. What's actually more consistent is the luteal phase, the gap between ovulation and your next period, which tends to stay around 14 days for most people.

So a more reliable way to estimate ovulation is to count back about 14 days from when your next period is due. If your cycle is 32 days long, that puts ovulation around day 18, not day 14. Our ovulation calculator does this for you and draws it on a simple cycle bar.

Signs to watch for

• Cervical mucus becomes clearer, stretchier and more slippery (a bit like raw egg white) in the run-up to ovulation.
• A slight rise in basal body temperature happens just after ovulation, useful for confirming it happened, less so for predicting it.
• Ovulation predictor kits detect the surge in luteinising hormone (LH) that triggers ovulation, usually giving you a day or so of warning.

If you're trying to conceive

Aim for the few days before and including ovulation. Having sex every one to two days across your fertile window is a common, low-stress approach, it keeps sperm present without you having to pinpoint the exact day. Give yourselves time, too: most couples conceive within a year of trying.

If you're trying to avoid pregnancy

Calendar estimates and apps are not reliable contraception on their own, cycles shift, ovulation can be early or late, and sperm survive for days. If you're avoiding pregnancy, use a method designed for it and talk to your GP or a sexual-health clinic about what suits you.

The bottom line

Your fertile window is about six days long and centres on ovulation, which usually falls roughly two weeks before your next period. Tracking your signs alongside a calculator gives you the clearest picture, but remember every cycle is a little different, and these are estimates, not guarantees.

This article is general information, not medical advice. For guidance about your own fertility or contraception, speak to your GP. The NHS has trusted information at nhs.uk.