New First Response Daily Ovulation Test takes the guesswork out of finding your peak fertility days because you begin testing immediately the day after your period ends. Then you can test daily for the next 20 days. Why test for LH surge? This test detects the rapid increase in luteinising hormone (LH surge) which normally occurs 24-36 hours before ovulation. Your 2 most fertile days begin with the LH surge. You are more likely to become pregnant if you have intercourse within 24-36 hours after you detect your LH surge