hCG promotes secondary ovulation and supplementary corpus luteum formation during early pregnancy in mares

This study evaluated whether the administration of human chorionic gonadotropin (hCG) during early pregnancy accelerates ovulatory dynamics and alters serum progesterone concentrations in mares with dominant follicles. Thirty pregnant embryo-recipient mares with follicles ≥35 mm during early gestation were randomly assigned to receive either 1500 IU hCG IV (hCG group; n = 17) or saline solution IV (control group; n = 13). Mares were monitored by serial ultrasonography to determine the ovulatory response and the interval to secondary ovulation. Serum progesterone concentrations were analyzed using two complementary approaches: a chronological post-treatment analysis and an analysis synchronized with ovulation timing. The proportion of mares that ovulated within 48 h after treatment was significantly higher in the hCG group than in the control group (82.4% vs. 0%; P < 0.0001). Likewise, the interval between treatment and secondary ovulation was shorter in hCG-treated mares (median: 1.7 days; IQR: 1.6-1.8) than in control mares (median: 22 days; IQR: 20-23; P < 0.0001). The interval between primary and secondary ovulation was also shorter in hCG-treated mares (median: 38 days; IQR: 34-43) than in control mares (median: 54 days; IQR: 51-60; P = 0.0008). The chronological post-treatment analysis revealed a significant group × time interaction for serum progesterone concentrations (P < 0.0001). When the analysis was synchronized with ovulation timing, progesterone concentrations increased similarly over time in both groups (time effect: P < 0.0001), with no significant group × time interaction (P = 0.28). These findings demonstrate that dominant follicles present during early pregnancy remain responsive to exogenous gonadotropic stimulation, allowing secondary ovulation and, consequently, the formation of supplementary corpora lutea. Although hCG administration altered the temporal pattern of serum progesterone concentrations by accelerating ovulatory events, post-ovulation progesterone dynamics remained equivalent between groups when the analysis was synchronized with ovulation timing.