This happens on rare occasion when the test sample somehow gets diluted, making the testing device fail to pick up the HCG in the blood. Some reasons for this happening are the testing chemicals dilute the sample and are unable to pick up the hormone, too much liquid placed on the device, or in ectopic pregnancy, the type of HCG released is different from the one in a normal pregnancy. If a pregnancy test is negative, it should be followed up by a physical examination and/or ultrasound for confirmation.