Incidence of spontaneous ultrasonographic regression of the corpus luteum in pregnant embryo recipients and broodmares: a field study.

Overview

• This study analyzes the frequency and characteristics of spontaneous corpus luteum regression (luteolysis) during early pregnancy in Thoroughbred broodmares and embryo recipient mares.
• It examines how luteolysis is linked to early embryonic loss, particularly in pregnancies involving in vitro-produced embryos, and identifies factors associated with increased risk.

Background

• Early embryonic loss (EEL) between Days 14 and 42 of pregnancy is a significant reproductive challenge in horses.
• The corpus luteum (CL) is a hormone-secreting structure on the ovary important for maintaining pregnancy by producing progesterone.
• Spontaneous luteolysis (regression of the CL) during early pregnancy can indicate failure of maternal recognition of pregnancy (MRP), which is critical for pregnancy maintenance.
• Embryo transfer techniques, including in vivo-derived (IVD) and in vitro-produced (IVP) embryos, are used in equine reproduction, but their influence on EEL and luteolysis is not fully understood.

Study Design

• Retrospective analysis of 2,078 pregnancies in Thoroughbred broodmares and 532 pregnancies in recipient mares carrying IVD or IVP embryos.
• Data included occurrence of spontaneous luteolysis and early embryonic loss, assessed via clinical and ultrasonographic monitoring.
• Diagnosis of luteolysis was based on transrectal ultrasonography evaluating CL size, echogenicity, vascularity, and inter-ovulatory interval (IOI).
• Some mares with luteolysis were treated with progesterone or altrenogest to attempt pregnancy maintenance.

Key Findings

• Incidence of early embryonic loss:
  • Broodmares: 8.0%
  • Recipients with IVD embryos: 8.7%
  • Recipients with IVP embryos: 13.5%
• Among pregnancies experiencing EEL, spontaneous luteolysis occurred in:
  • 18.7% of broodmares
  • 22.2% of IVD embryo recipients
  • 26.7% of IVP embryo recipients
• Overall luteolysis incidence was highest in IVP pregnancies (4.9%) compared to 1.7% in broodmares and 1.9% in IVD recipients, a statistically significant difference (P < 0.05).
• Ultrasonographic parameters assessing corpus luteum health helped diagnose luteolysis, noting changes in size, echo-density, and blood flow.
• Treatment with progesterone or altrenogest was performed in 34% of luteolysis cases, resulting in about 44% pregnancy maintenance by Day 42 among treated mares.

Risk Factors for Luteolysis

• Individual mare characteristics influenced luteolysis risk.
• Type of pregnancy was relevant, with IVP embryo pregnancies showing higher susceptibility.
• Timing of embryo transfer mattered: late embryo transfers (Day 9 or 10 post-ovulation) in recipients with IVD embryos were linked to increased luteolysis incidence.
• Embryo vesicle size alone was not a reliable predictor for spontaneous luteolysis, suggesting other factors contribute to pregnancy failure.

Conclusions and Implications

• Spontaneous luteolysis during early pregnancy is more common than previously recognized, especially in IVP embryo pregnancies.
• Failure of maternal recognition of pregnancy is multifactorial, not solely dependent on embryonic morphology or size.
• The study emphasizes the need for improved and earlier diagnostic tools to detect vulnerable pregnancies at risk of luteolysis and EEL.
• Management strategies, including hormonal supplementation, may help maintain pregnancies but are not universally effective.
• These insights are particularly relevant for embryo transfer programs and reproductive management in performance horse industries.