The incidence of ectopic pregnancy, strictly speaking ectopic pregnancy, is increasing year by year. With the application of high-resolution ultrasound technology, especially the prevalence of transvaginal ultrasonography, most cases can be diagnosed by ultrasound in a timely manner, which provides important information for timely clinical management and selection of appropriate interventions. However, most patients do not understand the limitations of ultrasound in the diagnosis of ectopic pregnancy. The following questions are frequently asked by sonographers: 1. Why is the pregnancy sac not detected on ultrasound despite a positive pregnancy test? A positive pregnancy test only indicates that a pregnancy has occurred, at which point the embryo can be implanted inside the uterus, outside the uterus, or the site of the pregnancy is uncertain. For pregnancies implanted inside or outside the uterus, when the value of human chorionic gonadotropin (β-HCG) in the blood is low, it is difficult to detect the gestational sac due to its small size, and it is impossible to determine the site of the pregnancy. Generally, when the value of β-HCG in the blood reaches more than 1,000 mIU/ml, the gestational sac increases in size, and ultrasound examination is easy to find out the site of the pregnancy. The site of pregnancy is not sure of the pregnancy is only manifested in a positive pregnancy test, ultrasound failed to find the gestational sac, at this time the clinical treatment can only be symptomatic and monitor the changes in HCG, 1-2 weeks and then ultrasound re-examination. 2.Why the diagnosis on the ultrasound report is often “the possibility of ectopic pregnancy”? Ultrasound only in the uterus outside the adnexal mass found in the yolk sac and embryo, and have regular original fetal heartbeat can be 100% sure of the diagnosis of ectopic pregnancy. However, most of the ectopic pregnancy embryos die in early stage, only manifested as an uneven mass in the adnexal area, the ultrasound manifestation is very similar to other diseases such as pelvic inflammation, tumors, etc., which must be combined with the HCG value and clinical manifestations in order to consider the ectopic pregnancy, therefore, ultrasound can only suggest the possibility of ectopic pregnancy rather than a definite diagnosis. If the patient does not have pregnancy-related clinical manifestations, such as menopause, vaginal bleeding, urine HCG positive, etc., adnexal mass is often suggested as inflammation or tumor. 3.Why the first ultrasound suggests intrauterine pregnancy, but the second ultrasound becomes ectopic pregnancy? About 10% of ectopic pregnancy due to the accumulation of blood in the uterine cavity in the uterine cavity formed similar to the gestational sac of the “false pregnancy sac”. At this time, the ectopic gestational sac due to its own slow development did not form a mass, so the ultrasound can not be detected, so the intrauterine “false gestational sac” can easily be mistaken for early pregnancy in the uterus. With the passage of time, a mass appears in the adnexal area and blood flows out of the uterine cavity, the diagnosis of ectopic pregnancy may be clear. Tip: Pregnancy is a progressive process and ultrasound is limited to a single point in time.