Objective To investigate the mechanism of anticoagulation protein defect in the pathogenesis of unexplained recurrent miscarriage. Methods Fifty-seven patients with a history of unexplained abortion were enrolled as the investigation group for tests of protein C

protein S

antithrombinⅢ（AT-Ⅲ）

as well as activated protein C resistance （APC-R）. The control group con-sisted of fifty healthy women with a history of normal pregnancy and delivery. Blood samples were obtained for measuring serum activity of protein C

protein S

AT-Ⅲ

and APC-R. Patients with positive APC-R were tested for factorⅤ（FⅤ） Lei-den gene mutation by PCR-RFLP method. Results Of the 57 patients

12 （21.1%）

1 （1.8%）

and 5 （8.8%） cases were found with protein S

protein C

and AT-Ⅲdeficiency respectively

and 13 （22.8%） cases with positive results of APC-R. Of the control group

no protein C or AT-Ⅲdeficiency was ever found

whereas 2 （4.0%） volunteers were presented with protein S deficiency and 3 （6.0%） with positive results of APC-R. No FⅤLeiden gene mutation was identified in all the patients with positive APC-R results. Late spontan-eous abortion cases had higher incidence of anticoagulation protein defect than the early cases. Conclusion Anticoagulation protein defect may play a role in the pathogenesis of fetal loss

especially for those occurr-ing in late stage of pregnancy.