How early can nonpregnant ewes be detected based on the blood flow of the corpus luteum?

The study aimed to characterize morpho-functional changes in corpus luteum (CL) in pregnant and non-pregnant sheep, propose diagnostic criteria for non-pregnancy, and assess diagnostic accuracy around the expected luteolysis period. Crossbred ewes (n = 34) underwent estrous synchronization, divided into G1 (unmated, n = 20) and G2 (mated, n = 14). The day of ovulation was considered as day 0 (D0). Luteal dynamic was monitored from D0 to D17, with blood samples collected every 72 h for progesterone (P4). Pregnancy was definitively diagnosed at day 30. Analyses included CL area (CLA), CL blood flow (CLBF), CLBF:CLA ratio, and serum P4. Statistical models considered group, day, and interactions. Accuracy for early pregnancy diagnosis (days 12-17) was assessed. CLBF:CLA ratio (14.3 +/- 8.1 vs. 20.3 +/- 10.7; 10.2 +/- 10.4 vs. 17.8 +/- 9.4; 1.3 +/- 4.1 vs. 16.0 +/- 8.2%), CLBF (0.41 +/- 0.3 vs. 0.49 +/- 0.3; 0.26 +/- 0.3 vs. 0.45 +/- 0.3; 0.06 +/- 0.2 vs. 0.34 +/- 0.2 cm(2)), and CLA (2.5 +/- 1.1 vs. 2.3 +/- 0.9; 1.9 +/-; 1.1 vs. 2.3 +/- 1.1; 0.9 +/- 1.2 vs. 2.0 +/- 1.2 cm(2)) differed (P < 0.05) between pregnant and nonpregnant ewes at days 13, 14, and 16 after ovulation, respectively. The early diagnosis of nonpregnancy achieved accuracy values > 90% from day 15 on for CLBF and CLBF:CLA ratio, but not for CLA. At this day, CLBF and CLBF:CLA ratio were circa 50-fold greater in pregnant than in nonpregnant ewes (p < 0.0001). No ewe classified as in the 1st or 2nd quartiles for CL endpoints or P4 concentration was confirmed as pregnant at day 30. In summary, the diagnosis of nonpregnancy based on the objective evaluation of CLBF could be performed in sheep as early as at day 15 after ovulation.