The effects of hCG administration on size and blood flow of the corpus luteum were studied in 10 mares. The size and blood flow of the corpus luteum were analysed with pictures gained by ultrasound. Three out of four mares in the group of 21 days pregnant lost their pregnancy after hCG injection. One mare had twice a second ovulation between 8 and 10 days post ovulation. The mean size and mean blood flow of the CL reduced significant during the experiment. At the time that measurement number 6 was taken the mean size of the corpus luteum (17541 ±6288) was significantly (p<0.05) decreased compared with the mean size recorded at the time of measurements number 1 (20276 ±6628), 2 (19900 ±6532), and 3 (19478 ±6209). The mean corpus luteum blood flow was at the day of hCG administration (measurements 1; 2999 ±1644, and measurement 2; 2900 ±1722) significantly (p<0.05) higher compared with the blood flow 3 days (measurement 4; 1626 ±1345), 5 days (measurement 5; 1502 ±1194) and 7 days (measurement 6; 1078 ±921) later. One day after hCG administration the mean corpus luteum blood flow (2440 ±1518) was significant (p<0.05) higher compared with the mean blood flow 7 days after hCG administration (1078 ±921). \ud If considering the different groups, the mares of group P-OV+21 had a significantly (p<0.05) smaller corpus luteum (12718 ±3827) during the experiment compared with the mares of groups P-OV+7 (20587 ±4512), NP-OV+7 (19826 ±6399) and NP-OV+14 (18718 ±6639). Moreover, although the difference was not significant, the corpora lutea of the group P-OV+21 mares seemed smaller than the ones of the group P-OV+14 (17165 ±6377, p=0.052). The corpus luteum blood flow in groups P-OV+14 (1391 ±1480) and P-OV+21 (1547 ±1198) mares was significantly (p<0.05) less than the blood flow in the mares of groups NP-OV+7 (2510 ±1671) and NP-OV+14. (2849 ±1999) The corpus luteum of mares of group P-OV+21 reduced significant (p<0.05) in size between the first (measurement 1; 15644 ±2787, measurement 2; 15516 ±2801) and the last day of measurement.(measurement 6; 9108 ±4463) In group P-OV+7 there was a decrease in size. The corpus luteum was at the time of measurement 3 (25744 ±2891) larger compared with the corpus luteum at measurement 6.(15668 ±2680, p<0.05) There was a significant (p<0.05) decrease in corpus luteum blood flow in group NP-OV+7 between the second (2501 ±1412) and the last measurement (526 ±403). \ud In this experiment we managed to show that hCG administration has a negative effect on the size and blood flow of the corpus luteum in cycling and early pregnant mares
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