A blood test for pregnancy is accurate. A blood test for HCG can find out if you are pregnant after the chorionic gonadotropin is elevated in the blood. Blood draws are more sensitive and can usually detect pregnancy about a week after embryo implantation. The clinical rule is that a pregnancy test can detect pregnancy two weeks after intercourse, and in some cases, it may be possible to detect pregnancy 6-7 days after implantation, so the blood test is very accurate. If the test paper is negative, the result of the blood test should prevail. In contrast, ultrasound may be late to check pregnancy, usually around 40 days after menopause, ultrasound may be able to see if there is a gestational sac in the uterine cavity. Therefore, if your period is delayed, the first thing to check should be blood HCG rather than ultrasound.