[Pregnancy and labor in the mare: uterine activity and endocrinology].

This research involved electromyographic recordings and hormone measurements in two pregnant mares during the last two weeks of their pregnancies. It found that uterine activity was greatest at night and increased in the 24 hours before parturition, falling right before delivery but spiking again at the rupture of fetal membranes. Meanwhile, changes in the ratio of two key hormones, estradiol and progesterone, may be related to labor occurring during daylight hours.

Research Methodology

• The researchers implanted four bipolar electromyographic (EMG) electrodes in the uteri of two pregnant mares. These electrodes were strategically placed near the tubo-uterine junction and bifurcation of the pregnant horn, in the body of the uterus, and near the cervix.
• Additionally, plasma samples were collected from the horses every four hours during the two-week period and more frequently during parturition (the act of giving birth).

Key Findings

• In the last week before delivery, uterine EMG activity was heightened, especially at night.
• A significant increase in this activity was noted in the 24-hour period preceding the birth of the foal. This lasted for 7 to 13 hours and was identified as stage I of parturition.
• Just 2 to 4 hours before birth, EMG activity dropped to very low levels. However, it sharply increased again with the rupture of the choriollantois – a combination of two fetal membranes in equines – and remained elevated till the delivery was complete. Afterwards, the activity decreased until the placenta was delivered.
• The ratio of hormones – estradiol 17 beta to progesterone – experienced an increase during the last week before parturition primarily due to a rise in estradiol 17 beta concentrations.
• In the 24 hours before birth, a significant reduction in progesterone led to a change in the hormone ratio.
• The levels of oxytocin and PGF2 alpha metabolite, another hormone, saw a sudden increase just before the rupture of the fetal membranes. A prior increase in oxytocin was observed before the PGF2 alpha metabolite.

Research Hypothesis

• The researchers hypothesize that the changing ratio of estradiol 17 beta to progesterone may be a factor influencing labor during the day. This could allow the evolution of labor to occur during daylight hours prior to parturition.