Clearblue® Advanced Digital Ovulation Test works differently to other ovulation tests as it is designed to detect 2 hormones, estrogen and LH. When it detects a rise in your level of estrogen it displays High Fertility (flashing smiley face), and will continue to display High Fertility in the following days whilst it looks for your LH surge. When your LH surge is detected, the test displays Peak Fertility (static smiley face).