Background: Threatened miscarriage is a common complication in the first trimester of pregnancy and is often associated with anxiety regarding pregnancy outcome. Approximately, 20% of pregnant women have symptoms of threatened abortion in the first trimester of pregnancy. A single serum cancer antigen 125 (CA-125) level determination is valuable in women with imminent abortion presenting with abdominal pain, vaginal bleeding or both rather than serial measurements. Objective: To evaluate the prognostic value of correlating the corpus luteum volume measured by two-dimensional ultrasound, and color doppler blood flow changes in the corpus luteum together with the results of serum CA-125 at the time of initial presentation with pregnancy outcome in patients with first trimesteric threatened abortion. Patients and methods: This prospective controlled study was held during the period from May 2015 to May 2018 on 100 women on their first trimester of pregnancy, 50 women having normal uncomplicated pregnancy and 50 women with clinical diagnosis of threatened abortion. The latter group was further subdivided into: group of patients who aborted (14 patients), and another group who continued their pregnancy and proceeded into the second trimester (36 patients). Results: It was found that the mean corpus luteum peak systolic velocity (CL PSV) for the threatened abortion (aborted) group was 30.68 ± 18.9 cm/sec, while for the threatened abortion (continued) group was 22.03 ± 12.9 cm/sec, and for the control group was 21.58 ± 13.77 cm/sec. The difference was statistically significant between threatened abortion (aborted) group and each of the threatened abortion (continued) group and the control group. The difference was statistically non-significant between the threatened abortion (continued) group, and the control group. The cut-off value of CL PSV at which there was a high probability of termination of pregnancy in threatened abortion cases, reached 17.785 cm/sec with a sensitivity of 63.6% and specificity of 54.4%. Conclusion: Serum CA-125 level determination was valuable in the women with symptoms of threatened abortion with a cut-off limit of 31.85 IU/ml of CA-125 level which carried a sensitivity of 86.4% and specificity of 80.7%. The study suggested that CA- 125 may be a cheap, sensitive and specific method in prediction of the cases of threatened abortion.