Once each month, a mature egg is released from one of the ovaries and is available to be fertilized. This 12- to 24-hour window is when your fertility peaks, making it the best time to have sex when you’re trying for a baby.

Ovulation test strips or ovulation predictor kits (OPKs) are at-home urine tests you can use to determine when you’re ovulating. Because you’re most fertile duringovulation, the kits can help increase your chances for success when you’re trying to conceive.

Ovulation test strips work by measuring levels of luteinizing hormone (LH) in your urine. A rise in LH signals the ovary to release an egg — so when your levels reach a certain threshold, it’s safe to assume that ovulation will occur within the next 12 to 36 hours.

How do you use ovulation strips to predict your most fertile days of the month?

All you have to do is pee on a stick or strip (or pee in a cup and dip the stick or strip into the cup) and wait a few minutes for the indicator to appear. Ovulation predictor kits have two lines.

  • One line is the control line. This just lets you know that the test is working.
  • The second line is the test line. When the test line is as dark as or darker than the control line, LH is surging and you’re about to ovulate. But if the test line shows up lighter or fainter than the control line, you’re not ovulating yet.

To get the most accurate reading, you’ll want to:

  • Use an ovulation test strip between 12 p.m. and 8 p.m. Most women have a surge in LH in the morning, and those levels can be picked up in your urine about four hours later. You could even test twice a day.
  • Make sure your urine is concentrated enough. Avoid peeing for an hour or two before the test, and try not to drink large amounts of fluid.

The kits also tend to be more accurate for some women than others. Ovulation predictor kits work best for women whose cycles are relatively predictable. They might be less reliable if you:

  • Have very irregular periods.
  • Have a health condition like PCOS.
  • Are taking certain medications.