INTRODUCTION: Abortion is the commonest complication of pregnancy and vaginal bleeding is the commonest complaint of patients within the first trimester of pregnancy. Obtaining assurance about the health of fetus heart and the product of pregnancy lowers the risk of abortion. Unfortunately, there is no reliable way of predicting the outcome of pregnancy in patients with threatened abortion. Levels of CA-125, a glycoprotein of coelomic origin, rise in the first trimester of pregnancy, especially in the presence of vaginal bleeding. This study was conducted to determine the efficacy of maternal serum CA-125 in predicting the outcome of pregnancy.METHODS: The extent, to which CA-125can serve as a predictor of the outcome of pregnancy, has been investigated in this study.CA-125 levels were measured in 100 women who were 8-12 weeks into their pregnancy and presented to the Emergency Services of Alzahra and Shahid Beheshti Hospitals with vaginal bleeding. The presence of fetal heart beat was demonstrated by ultrasonography. These women were followed up until the 20th week of pregnancy for abortion. Findings were analyzed with t-test and Kurscal-Wallis test using SPSS.RESULTS: The mean age of pregnancy and CA-125 level in the population under study was 9.54±1.32 weeks and 48.10±23.24Ulml, respectively. CA-125 levels in the abortion and nonabortion groups measured 73±24 U/ml and 42.4±17.52 U/ml, respectively, showing a statistically significant difference between the two groups. Eta correlation coefficient showed a strong correlation between elevated CA-125 levels and the likelihood of abortion. The CA-125 test had a sensitivity and specificity of 100% and 44.4%, respectively, at a concentration of 35 U/ml.DISCUSSION: Based on the findings of this study, CA-125 levels can be used to predict the outcome of pregnancy in patients with threatened abortion.