Transitory changes of hormones in the plasma of parturient pony mares

Frequent blood samples were collected from 8 pony mares before, during and after labour, parturition and placental expulsion and assayed for progesterone, oestradiol, androstenedione and LH concentrations by radioimmunoassay. A significant (P less than 0.05) decrease in progesterone, oestradiol and in the progesterone:oestradiol ratio was not detected until 0.5 h after foaling. Androstenedione concentrations rose before and peaked at parturition and then declined. A significant (P less than 0.05) rise in LH was detected 0.5 h after parturition. This LH peak was not detected in one mare and she was the only mare that did not ovulate within the first 20 days post partum. These results suggest that: (1) the foal may be an important factor in the production of progesterone and oestradiol by the feto-placental unit; (2) the pituitary is capable of releasing LH immediately after parturition; (3) the parturient rise in LH may be due to removal of negative feedback inhibition by progesterone and/or oestradiol; and (4) the parturient rise in LH at parturition, combined with already elevated concentrations of FSH, may be involved in the rapid growth of follicles post partum.