The main reason is that clinically it is necessary for the fertilized egg to be laid, either in the fallopian tube or in other parts of the body, and only after it has been laid can the trophoblast cells start to secrete blood HCG, and only at this time can the pregnancy be detected. It is best to take a blood test about 15 days after intercourse to find out if you are pregnant. If you want to use urine to test, it is recommended to use morning urine to test for early pregnancy about 20 days after intercourse, which may give a positive result.