How do I use an ovulation predictor kit?

Bottom lineUse an ovulation predictor kit by working out when you usually ovulate, then testing your urine daily for a few days beforehand at the same time each day with not-too-diluted urine; a positive (test line as dark as or darker than the control) signals an LH surge, meaning you'll likely ovulate within a day, making that day and the next your most fertile - though PCOS can cause misleading readings.

Ovulation predictor kits (OPKs) detect the surge in luteinizing hormone (LH) in your urine that happens about 24 to 36 hours before you ovulate. Used correctly, they help you find your most fertile days.

When to start testing

1. Know your cycle length so you can estimate ovulation
2. Begin testing a few days before you expect to ovulate (roughly cycle day 10 to 11 for a 28-day cycle, earlier for shorter cycles)
3. Test daily through your fertile window

How to test

• Test in the early afternoon to evening for many kits, and limit fluids beforehand so urine isn't diluted (follow your kit's instructions)
• Test at about the same time each day
• A positive means the test line is as dark as or darker than the control line

What to do with the result

A positive LH surge means you'll likely ovulate in the next day or so - your two most fertile days are the day of the positive and the day after.

Tips and limits

• PCOS and some conditions can cause misleading LH readings
• Kits show the surge but don't confirm an egg was released - pair with other signs if needed

Femora helps you time when to start testing by predicting your fertile window from your cycle history.

Find your window: ovulation calculator