If you test for pregnancy immediately after intercourse, it is usually not detected, and testing with a pregnancy test is even more inaccurate. A blood test for HCG must be taken 8-10 days after intercourse to determine if you are pregnant, which means that the earliest time to detect pregnancy is 8-10 days after sex, and it is impossible to determine if you are pregnant before then. The most accurate way to detect pregnancy is to take a blood test for HCG, which is called human chorionic gonadotropin, a protein hormone secreted by the trophoblast of the placental syncytium. It is only on the seventh day of pregnancy when the fertilized egg starts to lay that the placenta starts to secrete HCG. It is only on the eighth day of pregnancy, that is, the eighth day after the formation of the fertilized egg, that HCG can be detected in the mother’s blood, but at this time the HCG in the urine is often not detected with a pregnancy test and is still negative, but at this time it is a false negative because the pregnancy has indeed occurred. In order to detect HCG in urine, the HCG concentration in blood must reach a certain value. It is recommended to use a pregnancy test after seven days of postponement of menstruation in order to be more reliable.