Oestrogens, LH, PMSG, and prolactin in serum of pregnant mares

Levels of oestrone, equilin and equilenin (E1), oestradiol (E2) LH, PMSG and prolactin were measured by radioimmunoassay in serum from pregnant mares. Levels of E1 were always greater than those of E2. Both E1 and E2 remained at low levels until Day 80, increased significantly (P less than 0-05) by Day 120 to reach peak levels at Day 210 or 240 and then declined until parturition. Maximum levels of oestrogens observed in this study were 828 +/- 151 pg/ml for E1 and 71 +/- 18 pg/ml for E2 at Days 210 and 240 respectively. Spikes of LH release were observed in early pregnancy in most mares. Levels of PMSG were first detectable at Days 32 or 36 and reached a peak of 67-1 i.u./ml 64 days after the last mating. Levels of LH were very low (often non-detectable) from Day 210 of gestation until parturition. Levels of prolactin were extremely variable and showed no clear fluctuating patterns during pregnancy.