OBJECTIVE:To evaluate the predictive value of gestational age and maternal serum β-hCG concentration for the determination of the depth of trophoblastic invasion into the tubal wall. METHODS:This is a retrospective trial conducted on women with a diagnosis of ampullary pregnancy (71) who were submitted to salpingectomy. Serum β-hCG measurements were obtained at the initial admission of hospital. Histological investigation was performed by a single well-experienced pathologist who was blind to the clinical and laboratory characteristics of the patients. Ampullary pregnancy was classified histologically according to the depth of trophoblastic infiltration into tubal wall: trophoblast limited to the tubal mucosa (stage I), extended to muscularis layer (stage II) and complete tubal wall infiltration up to serosal layer (stage III). RESULTS:There was a significant difference in maternal serum β-hCG concentrations regarding the histological stages of trophoblastic invasion. The serum β-hCG concentrations that the best predicted for stage III trophoblastic invasion was 6,475 mIU/ml, with a sensitivity of 100 %, a specificity of 92 %. CONCLUSION:The depth of trophoblastic tissue infiltration into tubal wall is correlated with serum β-hCG levels, but not with gestational age. These findings may explain the reason for conservative management failure of EP in women with high β-hCG concentrations.