How do I know when I'm ovulating?

Almost everything about timing comes down to one fact that nobody leads with: the fertile window is mostly before ovulation, not after. Sperm can survive in the reproductive tract for up to about five days. An egg survives about twenty-four hours. So the days that count are the five days leading up to ovulation plus the day itself — six days, and then it's closed until next cycle.

Strong evidence That six-day window is one of the better-established findings in reproductive medicine, and it's the basis of ASRM's guidance. Which means the goal of tracking isn't to catch ovulation as it happens. It's to see it coming.

The four methods, and what each one is actually good for

Cycle apps: a notebook, not a predictor

Limited or mixed This is the one worth being straight about. A review of 73 fertility apps found they predicted the true day of ovulation only 33–43% of the time, and the "fertile windows" they draw — typically ten to sixteen days long — contained the actual ovulation day only about 65% of the time.

That's not because the apps are bad. It's because most of them are doing arithmetic on your past cycle lengths, and ovulation moves. The day of ovulation varies between cycles even in women with regular periods; what stays fairly constant is the second half of the cycle, not the first.

So use an app the way it's genuinely useful: to log periods, symptoms, test results and intercourse so you have real data over time, and to remind you when to start testing. Just don't let the little coloured window be the thing you plan around.

LH tests: the one that actually pinpoints

Moderate evidence Ovulation predictor kits detect the surge of luteinizing hormone that triggers ovulation roughly 24 to 36 hours later. That's the practical signal: a positive test means the window is open now and closing soon.

The digital ones with the smiley face solve a real problem. With cheap strips you're squinting at whether today's line is darker than yesterday's, which is genuinely hard and genuinely stressful. A digital reader makes the call for you. The dual-hormone versions (Clearblue's Advanced test is the common one) also measure estrogen, which rises earlier, so they give you a flashing smiley for "high fertility" days before the solid smiley for "peak" — that's the difference between two useful days and four or more. The manufacturer reports over 99% accuracy for detecting the LH surge itself.

How to use them without wasting sticks:

• Start a few days before you'd expect the surge. Subtract 17 from your shortest recent cycle length to find the day to begin.
• Don't use first morning urine. This is the opposite of a pregnancy test. LH usually surges in the early morning and takes a few hours to show up in urine, so late morning through early evening is better. Pick a time and keep it.
• Don't drink a lot right before. Dilute urine is the most common cause of a missed surge.
• A positive means go now, not tomorrow. The two days that matter most are the day of the surge and the day after.

Safety consideration Two honest caveats. A surge is not proof that an egg was released — it's the trigger, not the confirmation. And with PCOS, LH can run high or surge repeatedly, so these tests give false positives often enough that they're frustrating rather than useful. If that's you, this is worth raising with your doctor rather than buying more sticks.

Cervical mucus: free, and the earliest warning you get

Moderate evidence Secretions increase five to six days before ovulation and are at their wettest and most slippery two to three days before — right in the highest-probability part of the window. Clear, stretchy and slippery, often compared to raw egg white, means the window is open.

It costs nothing, it needs no supplies, and it starts signalling earlier than an LH test does. It's also the method most people were never told about. Used together, mucus tells you to start paying attention and an LH test tells you exactly when.

Basal body temperature: confirmation, not prediction

Insufficient evidence Your resting temperature rises a few tenths of a degree after ovulation, when progesterone kicks in. For timing sex this cycle, that's too late — by the time you see the shift, the window has closed.

What it's genuinely good for is answering a different question: am I ovulating at all, and roughly when? A few months of charting gives you a pattern to plan around, and a chart with no clear shift is useful information to bring to a doctor. Wearables that read temperature overnight (a ring, an armband, some watches) remove the part everyone hates, which is taking your temperature at the same time each morning before you move.

What this looks like in practice

Most people who track well end up doing some version of this, and it takes about a minute a day:

• Log the basics in an app so you have a cycle history and a reminder to start testing.
• Notice your mucus. When it turns clear and stretchy, the window is opening.
• Start LH tests a few days before you'd expect the surge, once a day, in the afternoon.
• Have sex through the window, not just on the smiley day. How often, and the timing mistakes that are easy to make →
• Optionally, temperature to confirm afterwards that ovulation happened.

Before you hand an app your data

Safety consideration Cycle apps hold unusually sensitive information, and the industry's record isn't clean. In January 2021 the FTC settled with Flo Health over allegations that the app had promised to keep health data private while disclosing it to third-party analytics and marketing firms — Facebook's and Google's analytics divisions, AppsFlyer and Flurry among them — including the fact that a user was pregnant. The settlement required Flo to get consent before sharing health data, notify affected users, and instruct those companies to destroy what they'd received.

That doesn't mean don't use an app. It means choose one deliberately. Worth checking before you start logging:

• Whether data is stored on your device or on the company's servers.
• Whether you can delete your account and your data, and what "delete" actually removes.
• What the privacy policy says about advertisers, analytics providers and "partners."
• Whether the app is a medical device cleared by the FDA, which a small number are, or a consumer app, which most are.

When irregular cycles change the answer

If your cycles are shorter than 21 days, longer than 35, or unpredictable enough that you can't guess when to start testing, that's worth a conversation rather than better tracking. Irregular cycles are the most common sign that ovulation isn't happening reliably, and guidelines support evaluating that at any age without waiting out the usual clock. When to ask for an evaluation →