There are several possibilities for a strong positive pregnancy test but no pregnancy: one possibility is that the pregnancy test is not working and although it is strong and positive, there is actually no pregnancy. Another possibility is that there is a problem with the way the pregnancy test is done. If the pregnancy test is strong and positive just a few days after menstruation has stopped, but an ultrasound reveals that there is no gestational sac in the uterine cavity to determine that there is no pregnancy, there is a problem with the way the test is done. Because in early pregnancy, pregnancy tests can be done by pregnancy test and blood HCG, while ultrasound can only show up after 10 days of pregnancy, so it is too early to do ultrasound to find out pregnancy. Therefore, it is not necessary to do ultrasound at this time, but to do a blood HCG test to determine if you are pregnant.