It is recommended that the pregnancy test should be used at least 15 days after intercourse. It takes 24 hours after ovulation for the sperm and egg to unite and form a fertilized egg, and 7-8 days after that for the embryo to be implanted, and only after the embryo is implanted can the change in HCG be detected in the urine and blood. Early low HCG can only be measured in the blood, not in the urine, and will only be measured after a certain value is reached, which is about 5-7 days later than the blood test. If the day of ovulation at the time of intercourse is more than 7 days, because occasionally we encounter sperm that have survived for 7 days, the sperm will be fertilized after ovulation and will not have fertilized on the 15th day after intercourse, which can also result in a false negative. This is why it is often the case that the urine test is negative, but you find out you are pregnant at a later stage. This may be related to the timing of the ovulation day calculation and the wrong timing.