The most reliable ultrasonographic criteria for diagnosing ectopic pregnancy are based on the appearance of the uterus in the presence of a positive urine pregnancy test. The absence of an intrauterine sac signifies ectopic pregnancy, whereas its presence indicates intrauterine gestation. These criteria cannot be applied when serum pregnancy tests are used unless the serum human chorionic gonadotropin (hCG) level at which the sac of an intrauterine pregnancy becomes visible on ultrasound is known, because these tests are far more sensitive and identify pregnancy at an earlier stage than does sonar. In this study, the hCG level that distinguishes patients with intrauterine pregnancies in whom a gestational sac can be seen from those in whom it cannot be seen was determined and designated the discriminatory hCG zone. This zone lies between 6000 and 65000 mIU/ml. The absence of an intrauterine sac in conjunction with hCG values above this level signifies ectopic pregnancy; however, the absence of an intrauterine sac has no diagnostic significance when associated with hCG values below the discriminatory zone. An intrauterine sac associated with hCG levels above the discriminatory zone reliably indicates an intrauterine pregnancy, but at hCG values below the zone, it is suggestive of an abnormal pregnancy-either a missed abortion or an ectopic gestation.