Progesterone in Early Equine Pregnancy
Study results indicate treatment with synthetic progestins early in pregnancy might be justified in mares with subphysiological progesterone concentration during the early luteal phase.
Two pregnant pony mares. Progesterone concentrations are important in early pregnancy for mares.
Reduced plasma progesterone concentration in early pregnancy results in increased early embryonic loss, indicating treatment with synthetic progestins might be appropriate. | Getty Images

A subphysiological plasma progesterone concentration in mares during early pregnancy might hinder development of the conceptus and endometrium until placentation. Researchers in Austria looked at how a mare’s low progesterone concentrations affect growth throughout gestation, embryonic and fetal loss, and foal characteristics at birth [Wagner, LH.; Aurich, J.; Melchert, M. et al. Low progesterone concentration in early pregnancy is detrimental to conceptus development and pregnancy outcome in horses. Animal Reproduction Science Sept 2023, vol. 257;]. 

Equine Progesterone Study

The two-season study involved Shetland mares aged 3-19 years following a thorough reproductive evaluation to ensure healthy status. Twelve mares were bred in the first year, and 14 mares were bred in the second year.  

Once inseminated, the mares were assigned randomly into two groups. One group of mares received PGF2alpha (62.5 μg analogue cloprostenol) for four days (Day 0 until Day 3) to reduce the luteal progesterone secretion during the post-ovulatory phase. The other group of mares in the control pregnancy group were untreated. The research team used transrectal ultrasound to evaluate conceptus size prior to Day 101 and abdominal ultrasound after Day 101. They obtained blood samples for measuring progesterone periodically throughout the study: daily until Day 15, every five days between Days 15-60, every two weeks between Days 60-280, and every 10 days between Day 280 and foaling. 

The same mares were bred the subsequent year and grouped opposite to their previous category, thereby serving as their own controls. The mares that had previously received cloprostenol now served as untreated controls; those that had been controls now received cloprostenol. 

Progesterone Profiles in Treated and Untreated Mares

Plasma progesterone in treated mares dropped during Days 3-14 following ovulation. After this two-week period, progesterone profiles were similar between treated and control pregnancies. The results are telling: 

  • Mares with subphysiological progesterone concentration experienced more pregnancy loss than the control mares.  
  • Mares with subphysiological progesterone concentration had smaller and lighter embryos in the early luteal phase (through Day 7) than the control mares. Foals of those pregnancies that continued to term were able to compensate for the initial reduced growth by the time of birth. 
  • Pregnancy rates on Day 12 post-insemination were lower in treated mares. All control mares retained their pregnancy by Day 12. 
  • A smaller conceptus size—related to subphysiological progesterone concentration—was more predisposed to pregnancy loss. Out of seven treated mares, two lost the pregnancy through early embryonic death or abortion and two foals were born compromised.  
  • The treated mares only foaled three noncompromised foals compared to all seven control mares birthing healthy, normal foals. Gestation length for all foals from both treated and control mares was 320 ± 2 days.  

Previous study results have determined that undisturbed luteal function during early pregnancy is critical to eliciting proper embryonic and endometrial gene expression. 

Final Thoughts

The authors summarized their study findings: “Reduced plasma progesterone concentration in the early luteal phase results in increased early embryonic loss and delays fetal growth beyond placentation. Treatment with synthetic progestins early in pregnancy may thus be justified in mares with reduced endogenous progesterone synthesis in the early luteal phase. It is therefore recommended to determine blood progesterone concentration on Day 5 after ovulation before such a treatment is initiated.” 

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