If you’re trying to conceive, you’re probably familiar with the basics of ovulation: Once each month, a mature egg is released from one of the ovaries and can be fertilized. This window lasts for less than 24 hours and is when your fertility peaks, making it the best time to have sex when you’re trying for a baby.
Ovulation predictor kits (OPKs) measure 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, though this can vary from woman to woman.
When using standard ovulation tests, begin testing several days before you expect your LH surge to begin. If your cycles are irregular, or you have no idea when you ovulate, you can start testing 5 days after your period ends. For the best results, it is recommended to have at least 10 test kits.