Variation in plasma concentrations of oestradiol-17 beta and their relationship to those of progesterone, 13,14-dihydro-15-keto-prostaglandin F-2 alpha and oxytocin across pregnancy and at parturition in pony mares.

The research article examines the variation in plasma concentrations of several hormones in pony mares during pregnancy and at the time of birth. It highlights that these hormonal changes seem more linked to external light conditions rather than the stage of pregnancy.

Hormone Levels During Pregnancy and Parturition

• The study found that plasma progesterone levels in the mares were mostly in line with what has been previously reported in literature, with the exception of a significant drop right before parturition detected through intensive blood sampling.
• Mean concentrations of oestradiol-17 beta, an estrogen steroid hormone, were observed to dramatically increase, then stabilize for around 133 days before sharply dropping in each mare. The timing of these increases and decreases showed considerable variation among the mares and were closely related to fluctuations in daylight rather than to the stage of pregnancy.
• The concentration of a certain form of prostaglandin F-2 alpha (PGFM) remained low until around day 200 of the pregnancy, after which it increased to peak levels by day 300 and maintained that level until parturition.
• Oxytocin levels were consistently low throughout the pregnancy, only increasing once the expulsive stage of labor started. This hormone is typically responsible for inducing labor and aiding in expulsion of the fetus.

Hormone Levels Pre-Parturition and During Parturition

• The study also noted an increase in the ratio of oestradiol-17 beta to progesterone starting from 3 days before parturition, reaching its highest point at delivery. However, this increase was not statistically significant.
• Noticeable acute changes occurred in the levels of PGFM and oxytocin during parturition. Maximal concentrations of both hormones were recorded between rupture of the chorioallantois (a fetal membrane involved in nutrition and excretion) and completion of delivery. Samples from one animal indicated that the increase in oxytocin levels predated any change in PGFM.
• In a mare experiencing dystocia (difficult birth), both oxytocin and PGFM peaked in the initial stages of delivery, but only the level of oxytocin remained high until the dystocia was resolved.

Conclusion

• The results suggest that an abrupt increase in oxytocin secretion triggers the expulsive phase of parturition in mares.