Research shows that you’re more likely to identify your LH surge if you start testing 19 days before your next period. Not sure how to figure out when it’s 19 days before your period? We’ve got you.

  • Start with day 1 of your period (your first day of full flow).
  • Next, determine your cycle length. Your cycle length is the total number of days from day 1 until your next period starts. It may take you a full cycle to count your cycle length, and that’s totally okay. 
  • Once you have the full length of your cycle, subtract 19 from the number to find your best day for testing. (If your cycle length is 28 days, you’ll want to start on day 9.) 
  • When you know your start day, test your LH levels for 10 days straight to find your LH surge. We recommend testing one additional day after your surge to confirm that you’ve found your peak LH level. 

The Modern Fertility App can also help you pinpoint when to start testing.

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