The possibility of distinguishing between normal intrauterine and ectopic pregnancies by determining the lower limit of the rate of human chorionic gonadotropin (hCG) increased in early pregnancy was investigated. This can be expressed as the slope of the log hCG-time curve or as the percent increase in hCG over a given sampling interval. For practical purposes, the rate is most easily determined from 2 samples drawn 48 hours apart. The differences between the 2 hCG values obtained is expressed as a percentage of the initial value, and should be 66% or greater for this sampling interval. Approximately 15% of normal intrauterine pregnancies screened in this way will appear abnormal, and the diagnosis in 13% of ectopic pregnancies will be delayed beyond 48 hours.