Abstract The present study was conducted during two distinct seasons on 118 Holstein cows in a dairy farm located in Ismailia Governorate, Egypt. Climatic data obtained from the Meteorological Authority in Cairo were used during the period from 2023 to 2024. All cows were subjected to the Ovi-synch protocol with fixed-time insemination. Transrectal ultrasonography was used to examine all cows. On days 10 to 13 after AI, the ovaries of all cows were carefully scanned for CL evaluation. Blood samples were collected on days 18-21 after the AI for progesterone assay. The mean CL diameter, area, and grayscale were significantly lower than their corresponding parameters recorded during winter. Furthermore, a clear negative correlation between THI and different CL parameters was observed. Summer conditions exerted a highly significant decline (P<0.001) in progesterone levels in cows that became pregnant compared to those in winter. In cows that failed to be pregnant, there was no significant difference (P > 0.05) in progesterone concentrations in cows of two groups. THI was negatively correlated with progesterone levels (P < 0.01), suggesting elevated THI characteristics of the summer season are associated with a decrease in serum progesterone levels. Progesterone concentration measured at the end of the cycle in cows that became pregnant showed a positive correlation with CL parameters. These correlations indicated that higher progesterone concentrations are associated with increased CL diameter, area, and volume, suggesting a potential relationship between elevated progesterone and enhanced luteal function.