Association Between Corpus Luteum Vascularization and Circulating Progesterone in Mares

Abstract Embryo transfer (ET) is used in the equine industry to maximize genetic potential, but success rates remain variable and are strongly influenced by circulating progesterone levels in recipient mares. Accurate monitoring of progesterone is essential for establishing an optimal uterine environment for embryo implantation; however, current hormone assays are time-consuming and costly, limiting their use in large-scale ET programs. This study aimed to evaluate the relationship between corpus luteum (CL) blood perfusion and circulating progesterone concentrations from d 0 to d 6 post-ovulation, and to assess the potential of Doppler ultrasonography as a non-invasive tool for estimating progesterone status in mares. A total of 23 mature, open, multiparous light-horse mares (BCS 5.6 ± 0.2) were used. Mares were housed at Valley Equine Reproduction (Stephenville, TX) and Tarleton State University. Blood samples were collected on d 0, 2, 4, and 6 post-ovulation via jugular venipuncture; serum was separated, stored at − 80 °C, and analyzed for progesterone by radioimmunoassay (Texas A P 0.10. Location was removed from the model after showing no effect. Progesterone concentrations increased from d 0 through d 4 (P ≤ 0.01) and plateaued thereafter, with differences between d 0 and all subsequent days and between d 2 and d 4 and 6. CL vascularization followed a similar pattern (P ≤ 0.01), increasing from d 0 to d 4 and 6, and from d 2 to d 4 and 6. Correlation analysis showed a moderate positive relationship between CL vascularization and progesterone (Pearson r = 0.495; Spearman r = 0.517; both P ≤ 0.01). Linear regression indicated that CL vascularization predicted progesterone concentrations (P ≤ 0.01), explaining about 25% of the variance (R² = 0.245). Each unit increase in vascularized area corresponded to an estimated 0.15 ng/mL rise in progesterone. These findings demonstrate that Doppler-derived CL vascularization is moderately correlated with circulating progesterone and may serve as a practical, non-invasive tool for assessing luteal function in mares. Further research should explore its application in optimizing recipient mare selection to enhance embryo transfer outcomes.