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

Overview

• This study investigated whether administering human chorionic gonadotropin (hCG) to pregnant mares with large ovarian follicles during early pregnancy could induce earlier secondary ovulation and influence progesterone levels.
• The research found that hCG significantly accelerated secondary ovulation and the formation of additional corpus luteum, although post-ovulation progesterone dynamics were similar when aligned with ovulation timing.

Study Objectives

• To evaluate if hCG administration during early pregnancy can promote secondary ovulation in mares with dominant follicles (≥35 mm).
• To assess whether hCG affects serum progesterone concentrations, which are vital for maintaining pregnancy.

Methodology

• Subjects: 30 pregnant embryo-recipient mares displaying dominant follicles ≥35 mm early in gestation.
• Groups: Mares randomly assigned to either:
  • hCG treatment group (1500 IU intravenously; n = 17)
  • Control group receiving saline solution intravenously (n = 13)
• Monitoring: Serial ultrasonography used to observe ovulation timing and secondary ovulatory events.
• Hormonal analysis: Serum progesterone measured using two approaches:
  • Chronological post-treatment analysis (measuring changes over time after treatment)
  • Analysis synchronized with the exact timing of ovulation to assess progesterone changes post-ovulation

Key Findings

• Ovulation rates:
  • 82.4% of mares in the hCG group ovulated within 48 hours post-treatment, compared to 0% in the control group.
• Timing differences:
  • Median interval from treatment to secondary ovulation was 1.7 days in hCG-treated mares versus 22 days in controls.
  • Interval between primary and secondary ovulation was also significantly shorter in hCG-treated mares (median 38 days) than controls (median 54 days).
• Progesterone changes:
  • Using chronological post-treatment analysis, a significant interaction between treatment group and time was found, indicating that hCG altered progesterone levels over time.
  • When progesterone data was synchronized with the timing of ovulation, both groups showed similar increases, with no significant difference in progesterone dynamics post-ovulation.

Conclusions

• Dominant follicles during early pregnancy in mares remain responsive to exogenous gonadotropic stimulation via hCG.
• hCG administration effectively accelerates secondary ovulation, leading to the formation of supplementary corpora lutea, which are structures that produce progesterone to support pregnancy.
• Although hCG changed the timing and pattern of serum progesterone concentration when viewed from treatment initiation, the overall progesterone profile following ovulation was comparable between treated and control mares.
• These results highlight the potential for hCG to regulate ovarian function and support early pregnancy physiology in mares by inducing secondary ovulatory events and enhancing luteal function.