Serum hCG and free beta hCG subunit were measured in intrauterine pregnancies (n = 21), ectopic pregnancies (n = 20) and spontaneous abortions (n = 19) matched for gestational age. Significantly higher concentrations of both dimer and free beta subunit hCG were detected in normal pregnancies (86,336 IU/L) and 21.02 IU/L respectively) compared to abortions (10,460 IU/L and 3.73 IU/L) and ectopic pregnancies (3,900 IU/L and 3.73 IU/L) (p less than 0.05). When the ratio of free beta hCG/intact hCG (%) was studied, however, EP had significantly higher ratios (0.09 ± 0.09) than IUP (p less than .05). Assessment of the relative distributions of these ratios revealed that 100% of IUP and AB and 65% of EP had ratios less than 0.10. Most notably, 35% of EP were uniquely characterized by ratios greater than 0.10. This ratio was sufficiently higher in 35% of EP to define a profile completely unique to EP. These data suggest that an increased free beta to dimer hCG ratio of greater than 0.10 may assist in the differentiation of ectopic from intrauterine pregnancies of spontaneous abortions and provide insight into a possible trophoblastic mechanics in these clinical events.