Trying to Conceive: Your Ovulation Calendar. Many couples rely on ovulation predictor kits, or OPKs, to identify when the woman is most fertile. OPKs can detect a surge in the production of luteinizing hormone (LH), which occurs roughly 36 hours before ovulation. By timing intercourse shortly after the LH surge, you can increase your odds of bringing sperm and egg together at the time when conception is most likely to occur.