Relationship between bovine endometrial thickness and plasma progesterone and estradiol concentrations in natural and induced estrus.

The objective of this study was to investigate cyclical changes in endometrial thickness in relation to progesterone (P ₄ ) and estradiol-17β (E ₂ ) concentrations during natural and induced estrus in 15 cows. In the prostaglandin (PG) F _2α -induced estrus group, ultrasonography (USG) at 6-h intervals was used to determine endometrial thickness 48-24 h before the PGF _2α treatment until 24 h after ovulation (ovulation = Day 0). In the natural estrus group, USG was performed every 48 h from Day 3 to Days 15-18 after the first ovulation, and then every 6 h until 24 h after ovulation. Endometrial thickness was standardized using Day 13 as a reference day. Blood was collected during every USG examination and plasma P ₄ and E ₂ concentrations were determined. Endometrial thickness of the induced estrus group (n = 11) was greater than that of the natural estrus group (n = 9) between 60 and 12 h before ovulation (P < 0.05). In the natural estrus group, prior to an increase in endometrial thickness, a decrease in P ₄ and an increase in E ₂ were detected. In the induced estrus group, based on the time of ovulation, an increase in endometrial thickness was detected at the same time of a decrease in P ₄ before an increase in E ₂ . These results suggest that decreases in P ₄ concentrations may be a cue to changes in endometrial thickness, while increases in E ₂ concentrations appear to sustain and/or enhance these changes.