The earliest way to know if you are pregnant is more than 7 days after intercourse. There are two ways to check if you are pregnant after sex, one is to check your urine and the other is to check your blood. But whether it is blood or urine, the test is to detect changes in human chorionic gonadotropin, or HCG, in the body. Changes in human chorionic gonadotropin, or HCG, in the blood are more noticeable and faster than changes in HCG in the urine, and can be detected in the blood usually more than a week after intercourse. The accumulation of HCG in urine is relatively slow, and there is a clear relationship with the time of holding urine, so the accuracy of HCG in blood is relatively high. So if you want to determine whether you are pregnant or not, you can determine whether you are pregnant or not by blood test in the hospital more than one week after intercourse.