An ovulation test picks up a surge of LH (luteinising hormone) in your urine, and ovulation usually follows a day to a day and a half later. Your most fertile days are the day the test turns positive and the day after. Used exactly as the instructions say, these tests find the surge reliably about 9 times out of 10.
When should I start?
Start 3–5 days before you expect to ovulate: with a 28-day cycle, that's around day 11. For other cycle lengths the leaflet has a table, and if your cycle varies, use your shortest cycle from the last six months. Then test every day until you get a positive, because skipping a day can mean missing the surge.
Morning or afternoon?
This is where brands disagree. Some ask for the first urine after sleep, others suggest late morning to evening, because a surge may not show up in morning urine yet. Follow what's written on your box, test at the same time each day, and don't drink a lot beforehand.
How do I read the strip?
A faint second line shows up almost every time, because your body always makes a little LH. On a standard strip, a positive means the test line is as dark as the control line or darker. Read it within the time the leaflet gives, not later, and if there's no control line, the test hasn't worked.
Strips or digital?
Plain strips pick up the surge just as well as digital tests. A digital test costs more but spells out the answer, so there are no lines to compare. Some tests also pick up a breakdown product of oestrogen, which builds up in urine around ovulation.
It's positive. Now what?
For getting pregnant, the best time is the day of the positive and the day after. Sex in the few days before the positive counts too, since sperm usually live for up to 5 days, sometimes up to 7. And if testing wears you out, you can drop it: sex every 2–3 days throughout the cycle gives you a good chance anyway.
Common questions
How many tests will I use per cycle?
A kit usually has 5–7. You can stop after the first positive in a cycle, and with irregular cycles you'll usually need more.
The second line is always faint. Is something wrong?
Probably not. It's easy to miss the surge if you skip a day or start late. If you never get a positive, cycle after cycle, show your notes to a doctor.
I got a positive. Does that mean I ovulated?
Not for sure. The test only sees the LH surge and can't tell whether an egg was released. A progesterone blood test about a week before your period can check that, if a doctor orders one.
Can I use ovulation tests as contraception?
No. The positive shows up only a day to a day and a half before ovulation, and sperm from sex a few days earlier can still be alive.
In short
- Start 3–5 days before you expect to ovulate
- Test every day at the same time until the first positive
- Sex every 2–3 days works even without tests
Source: FDA: Ovulation (urine test)
Educational content; it does not replace professional medical advice.