tests can give variable results at missed period as they measure the hCG
levels which may not be higher enough to be detected at this stage. Serum Beta-hCG levels give a more accurate result. They can detect levels of even 1 mIU/ml at around the missed period. The projected levels of hCG can detect an ectopic pregnancy, as normal values peak at 10-12 weeks of pregnancy before they plateau and fall.The gestation sac is usually seen at about 4-5 weeks of pregnancy by trans vaginal ultrasound
if the hCG levels are around 1500 mIU/ml. If the gestation sac is not formed even after the second sonogram and if ectopic pregnancy is ruled out, it could be a false test or a missed abortion
. Kindly consult your doctor for further opinion. Take care.