Endocrine aspects of early pregnancy in pony mares: a comparison of uterine luminal and peripheral plasma levels of steroids during the estrous cycle and early pregnancy.

This research investigates the hormone levels in the blood and uterine fluid of pony mares during their estrous cycle and early pregnancy, suggesting significant hormonal changes in the uterine environment which are not detectable in the peripheral blood.

Research Design and Methodology

• The study involved examining the concentrations of three specific hormones – estrone, 17 beta-estradiol, and plasma progestin – in both the uterine fluid and peripheral blood of non-pregnant and pregnant pony mares.
• A total of 30 mares were divided into six groups. Each group’s uterine fluid and peripheral blood were sampled on specific days post-ovulation (days 8, 12, 14, 16, 18, and 20).
• Furthermore, each mare’s uterine environment was further examined by measuring hormone levels within the yolk sac fluid from blastocysts on days 12, 14, 16, and 18 of pregnancy.
• Following a recovery period of one estrous cycle, the mares were bred. The animals were then hysterectomised on the same treatment day to which they had previously been assigned in the nonpregnant phase of the study.

Key Findings

• Significant differences were found in the day-to-day trends between nonpregnant and pregnant animals for both estrogens and progestins in both uterine fluid and peripheral plasma.
• The research data indicated that notable increases in estrogen levels occur after day 12 of pregnancy in both the uterine and yolk sac fluids. By days 18 and 20, estrone became the dominating estrogen in yolk sac and uterine fluids, respectively.
• These observed changes could not be detected in the peripheral blood, suggesting that the hormonal changes happening inside the uterus during early pregnancy are not discernible in the overall blood circulation.

Research Implications

• The findings suggest that the large amounts of estrogens observed in the uterine fluids during early pregnancy originate from the conceptus (the embryo and the tissues that support it).
• This discovery could significantly impact the understanding and management of early pregnancy in horses, as these hormones may play a vital role in regulating the developing embryo’s environment.
• As this research only involves pony mares, further studies might need to include different horse breeds and varieties to confirm whether these findings apply universally.