Investigations into the course of progesterone-, oestrogen- and eCG-concentrations during normal and impaired pregnancy in the mare

Blood plasma samples were collected weekly after mating from 38 mares and a comprehensive picture covering the whole period of pregnancy was obtained for progesterone, free and conjugated estrone equivalents, eCG and cortisol. The appearance and disappearance of eCG between weeks 5 and 20 was correlated with the formation and regression of supplementary corpora lutea; the progesterone plateau observed thereafter and the increase prior to parturition were attributed to cross-reactivity of the antiserum used with other progestagens. Conjugated estrone equivalents showed a first increase concomitant with the formation of supplementary corpora lutea, the second increase commenced during weeks 10-12, reaching maximum levels around 700 ng/ml plasma in week 22. Free estrone equivalents did not start to increase before week 14. Progestagens and estrogens showed a precipitous decline one day prior to parturition while cortisol did not change; however, cortisol levels were higher (p < 0.001) during weeks 10-17 compared to the other periods of pregnancy. Additionally, in two mares an extended period of eCG production until parturition or abortion (week 42) was observed. In cases of embryonic resorptions the levels of progesterone and conjugated estrone equivalents decreased rapidly, eCG levels, however, remained elevated and apparently did not interfere with resumption of cyclic ovarian activity. Compared to normal pregnancies, the progestagen and estrogen levels were higher between week 22 and the week prior to abortion (week 31-42) in the five aborting mares. In three mares defined as pseudopregnant, luteal function was extended up to 77 days after mating but no rise of conjugated estrone equivalents or eCG occurred.