Measurement of serial human chorionic gonadotropin (hCG) concentrations facilitates the diagnosis of normal or abnormal early gestation, as deviations from expected rates of rise or decline prompt intervention and/or treatment of abnormal pregnancies. Recently, the expected rise in hCG levels for normal intrauterine pregnancies (IUPs) was well characterized, and based on a 95% confidence interval, it was determined that the slowest rise in a normal IUP was 53% in 2 days. When the hCG level is decreasing, it is clear that the pregnancy is not viable. The management of such cases consists of observation or intervention. The rate of decline associated with nonviable pregnancies that resolved with expectant management has been described, with a faster decline observed with higher hCG levels at presentation. A rate of decline that is slower than 21 to 35% in 48 hours suggests the presence of retained trophoblastic tissue or an ectopic pregnancy and indicates the need for intervention to achieve a definitive diagnosis. While there is no consistent pattern of serial hCG levels characterizing ectopic pregnancies, the majority of patients ultimately diagnosed with ectopic pregnancy will exhibit rates of rise or decline that are outside the "normal" range as defined by the established curves.