Cortisol and progestin release, heart rate and heart rate variability in the pregnant and postpartum mare, fetus and newborn foal.

This research focused on the mechanisms leading to horse childbirth, specifically the role and changes in cortisol and progestin hormones, as well as alterations in heart rate and heart rate variability in the mother, fetus, and newborn horse. The researchers hypothesized an increase in maternal cortisol before childbirth, which can be identified in both saliva and plasma. The study analyzed heart rate and fluctuations in heart rate and used these findings to discern a change in cardiovascular demands resulting from the transition to life outside of the womb.

Overview of Study Methodology and Design

• The study population involved 25 pregnant mares. Out of this group, 13 mares were selected to also undergo fetomaternal electrocardiogram (ECG) recordings.
• Saliva samples were collected from the mares four times a day, from 15 days prior to 5 days after foaling.
• In the subset of 13 mares, blood samples were also taken daily for progestin and cortisol analysis.
• ECG recordings were used to document heart rate (HR) until five days post-foaling. From this data, researchers analyzed heart rate variability (HRV), calculating the standard deviation of the beat-to-beat interval (SDRR) and the root mean square of successive RR differences (RMSSD).

Key Findings and Observations

• From 15 to 4 days prior to labor, the concentration of progestin significantly rose, peaking at 267 ± 42 ng/mL, and dropped thereafter. The progestin level on the day of foaling was lower at 113 ± 18 ng/mL.
• A notable rise in maternal cortisol concentrations was recorded in the blood and saliva before childbirth, coinciding with the decline in progestin concentration.
• The mares’ heart rates did not change during the final days of pregnancy but decreased within the first day after labor. However, maternal HRV did not exhibit any changes.
• In the fetus and neonate, the heart rate increased from before to after birth. This suggested escalating cardiovascular demand as the newborn adjusts to life outside the womb.

Conclusion

The research indicates that cortisol levels in pregnant mares increase prior to childbirth, as reflected in both blood and saliva. These cortisol changes are associated with changes in progestin levels. Moreover, changes in heart rate in mares and their offsprings reflect the different cardiovascular demands before, during, and after birth.