A blood test is done 10 days after intercourse to determine if you are pregnant, and the main test is for human chorionic gonadotropin (HCG). So if you had intercourse on the 3rd, you will need to take a blood test on the 13th to check for human chorionic gonadotropin to determine if you are pregnant. To check with pregnancy test, you need to take a blood test on the 17th, two weeks after intercourse. However, the value of human chorionic gonadotropin varies from person to person. If you do not detect pregnancy, you can observe for a period of time until your period has been delayed for more than 7 days and you have not yet had your period, and then take the test again, for a higher rate of accuracy. If menstruation is delayed for more than 10 days, you can do ultrasound examination, ultrasound examination can determine whether it is intrauterine pregnancy or ectopic pregnancy, and at the same time, the size of the gestational sac, the size of the fetus can be found.