Progesterone, prostaglandin F-2 alpha, PMSG and oestrone sulphate during early pregnancy in the mare.

Blood samples from 4 mares during the late luteal phase, oestrus, early pregnancy and up to about 150 days of gestation were analysed for 15-keto-13,14-dihydroprostaglandin F-2 alpha (PGFM), progesterone, PMSG and oestrone sulphate by radioimmunoassays. During the late luteal phase, at the time of corpus luteum regression and decreasing progesterone levels, PGFM peaks were recorded. During early pregnancy (i.e. from mating and up to about Day 30) no such peaks were detected. After mating the progesterone levels increased and remained high throughout the observation period. During the oestrous cycle PMSG levels tended to reflect gonadotrophic activity because of high cross-reaction with horse LH and horse FSH. During early pregnancy PMSG levels increased at around Day 40, reaching maximum values between Days 50 and 80, when the blood concentration was 1000 times higher than values during oestrus. Oestrone sulphate concentrations increased about 3-fold during oestrus and about 10-fold at the peak of PMSG. After about Days 70-75 of pregnancy the levels of oestrone sulphate increased further and values of greater than 250 nmol/l were reached around Day 100.