We aimed to identify the functional characteristics of the corpus luteum (CL) by color Doppler ultrasonography and changes in interferon-stimulated gene (ISG) expression in peripheral blood mononuclear cells (PBMCs) during early pregnancy in beef cows. We then aimed to use these features to establish earlier pregnancy diagnosis methods. In experiment 1, the CL size and blood flow were accessed by Doppler ultrasonography, and the PBMCs were isolated on Days 8, 12, 15, 18, 20, 22, 25, 30, 45, and 60 post-timed artificial insemination (TAI) from pregnant (n = 10) and nonpregnant cows (n = 12). The abundance of ISG (OAS1, MX1, MX2, and ISG15) transcripts was measured by quantitative PCR. Analyses of OAS1 and MX2 expression in isolated PBMCs (ISG-PBMC method) and Doppler imaging of CL (Doppler-US method) were performed to test the accuracy of these methods for the diagnosis of pregnancy on Day 20 post-TAI (n = 110; experiment 2). In experiment 1, the luteal volume and blood flow were reduced in nonpregnant cows during the first weeks post-TAI, but an evaluation of CL vascularization and size was efficient in identifying nonpregnant cows on Day 20 post-TAI. The expression of ISGs in PBMCs can be stimulated by the presence of a viable conceptus between Days 15 and 22 post-TAI, and the expression of these genes reaches a peak on Day 20. In experiment 2, the Doppler-US and ISG-PBMC methods resulted in similar specificity (85.5 and 87.7%, respectively). However, only the Doppler-US method resulted in 100% sensitivity. In conclusion, the greatest abundance of ISGs in PBMCs and a high detection of luteolysis by Doppler imaging on Day 20 post-TAI can be feasibly used for the earlier detection of nonpregnant cows in reproductive programs. The level of accuracy for our described pregnancy methods is high on Day 20 (80%–91%), but only the Doppler imaging method results in an absence of false-negative diagnoses.