The reasons for this development of events may be as follows:
1. You had an early ultrasound. It should be carried out with a vaginal datchik, starting from the 3-4-5th day of the absence of menstruation on time, and with late ovulation, these terms are shifted upwards.
2. There is an ectopic pregnancy, which is not yet visualized with ultrasound scanning. A blood test for hCG with an interval of 2-3 days and a second ultrasound will help clarify the situation; its level should increase. An increase in hCG values in blood tests and the absence of a fetal egg in the uterine cavity testifies in favor of an ectopic.
3. The body has a source of production of the hormone chorionic gonadotropin, which gives positive results of a test or blood test. This happens with such a pathology of pregnancy as cystic drift, as well as with some tumor processes. A retake of the analysis, repeated ultrasounds in dynamics and consultation of a specialized specialist are required.