Transitory changes of hormones in the plasma of parturient pony mares.

This research studied hormone changes in pregnant pony mares before, during, and after giving birth. The researchers noted significant changes in certain hormones, suggesting that the foal and a particular hormone (LH) may play key roles in the pony mares’ reproductive processes surrounding birth.

Methodology

• The study included blood samples obtained frequently from eight pony mares before, during, and after labour, birth (parturition) and the expulsion of the placenta.
• The blood samples were tested (via radioimmunoassay) for various hormone concentrations: progesterone, oestradiol, androstenedione, and LH.

Findings

• A significant drop in hormone levels—progesterone, oestradiol, and the ratio of both these hormones—was noted, but it was only detected half an hour after foaling.
• The androstenedione hormone showed an increase prior to birth, peaking at parturition, and then declining subsequently.
• A significant spike in LH, half an hour after birth, was observed. One mare did not show this LH peak, and it was uniquely the mare that did not ovulate within twenty days after giving birth.

Implications

• The study suggests that the foal could play a crucial role in the production of progesterone and oestradiol by the feto-placental unit. This assumption is based on the significant decrease in these hormones after the foal is born.
• The pituitary gland in the mares can release LH immediately after parturition as evidenced by a marked rise in LH 30 minutes after birth. The rise might be a response to the lifting of negative feedback inhibition by progesterone and/or oestradiol.
• The study postulates that the rise in LH at birth, along with already high FSH (follicle-stimulating hormone) levels, may contribute to rapid follicle growth after birth. This conclusion is supported by the observed association between the LH increase and ovulation in mares post-parturition.