How ovulation predictor kits work
Ovulation predictor kits (OPKs) are urine tests that detect luteinising hormone (LH). LH stays low for most of the cycle and then rises sharply — the LH surge — about a day or a day and a half before an egg is released. A positive test therefore gives advance warning that ovulation is close, which is exactly when sex is most likely to lead to pregnancy.
The challenge is timing. Test too late and you can miss a short surge; start too early and you use up tests. This calculator works out when to start for your cycle length and lays out a daily plan through the days your surge is most likely.
Which day to start testing
Because ovulation usually happens about 14 days before the next period, the common guidance is to start testing about 17 days before your next period is due:
| Cycle length | Start testing on cycle day |
|---|---|
| 24 days | 7 |
| 26 days | 9 |
| 28 days | 11 |
| 30 days | 13 |
| 32 days | 15 |
| 35 days | 18 |
Cycle day 1 is the first day of full bleeding. If your cycle length changes from month to month, use your shortest recent cycle so you don’t miss an early surge.
Reading the result
- Line tests: the test is positive only when the test line is as dark as or darker than the control line. A faint test line is normal — LH is always present at low levels.
- Digital tests: show a clear symbol, such as a smiley face, when the surge is detected; some also detect rising oestrogen beforehand.
- Once you get a positive, you can stop testing for that cycle. The best days for sex are the day of the positive and the next two days.
Tips for better results
- Test at about the same time each day and avoid drinking a lot of fluid for two hours beforehand, which can dilute urine.
- Some people have a short surge; testing twice a day around the expected surge days can help.
- Conditions such as polycystic ovary syndrome (PCOS), recent pregnancy, perimenopause and some fertility medicines can cause misleading results — a doctor can advise.
- Combine tests with other signs, such as egg-white cervical mucus, for a fuller picture.
If you get no positive test after several cycles, or your cycles are very irregular, talk to a healthcare professional. To see your fertile window without tests, use the Ovulation Calculator, and when it is time to check for pregnancy, the Pregnancy Test Calculator shows the best day.