Objective: To determine whether clinically routine clotting tests such as activated partial thromboplastin time (aPTT), prothrombin time (PT), or fibrinogen can be used to predict further miscarriages.Design: Prospective study.Setting: Nagoya City University Hospital, Nagoya, Japan.Patient(s): A total of 261 patients with a history of two consecutive first-trimester spontaneous abortions who had no antiphospholipid antibodies or other autoimmune diseases and no anatomic anomalies were examined for aPTT, PT, and fibrinogen before becoming pregnant again.Intervention(s): Blood tests were performed before pregnancy. Patients then were followed up during subsequent pregnancy and their outcomes were compared with their previous blood test results.Main Outcome Measure(s): Activated partial thromboplastin time, PT, and fibrinogen were measured by coagulation time methods.Result(s): Fifty-eight of 261 patients (22.2%) had a subsequent miscarriage. Mean (+/-SD) values for preconception aPTT in individuals whose subsequent pregnancies ended in success and failure were 88.2% +/- 23.4% and 99.3% +/- 26.4%, respectively. The difference was statistically significant. Respective values were 106.8% +/- 22.8% and 106.3% +/- 21.4% for PT and 245 +/- 61.1 mg/dL and 259.1 +/- 57 mg/dL for fibrinogen. These findings were not significantly different.Conclusion(s): A shortened aPTT before conception is associated with further miscarriages in patients with a history of recurrent spontaneous abortions who have no antiphospholipid antibodies.