Your most fertile days can change from woman to woman & cycle to cycle... The Clearblue Digital Ovulation Test detects a hormonal surge (LH) in your urine, which occurs prior to ovulation. Make love at any time in the next 48 hours to maximise your chance of becoming pregnant. Detection of LH surge is proven to be amore accurate predictor of ovulation than temperature, which only changes after ovulation. Over 99% accurate: No test is more accurate, proven in extensive laboratory tests. Easy to use: With 20 tests there is no need to know your usual cycle length. Just start testing 6 days after your period starts, until you pick up your LH surge (indicated by a smiley face). Most accurate reading: With its digital display, it is the clearest ovulation test to read.