The main reason is that on the day of intercourse, the sperm and egg are able to unite and a fertilized egg is formed. On average, the fertilized egg takes 12-13 days to enter the uterine cavity, after which it takes 2-3 days for the trophoblast cells to start to secrete HCG, so from this time, i.e. 15 days from the time of intercourse, the blood HCG can detect whether you are pregnant. If you want to test by urine HCG, it is better to test after 35 days of menopause.