Progestagen profiles during the last trimester of gestation in Thoroughbred mares with normal or compromised pregnancies

Progesterone (P4), pregnenolone (P5) and their metabolites are present in maternal plasma in pregnant mares. It is believed that one of these progestagens may maintain myometrial quiescence. The aims of this study were to identify specific progestagens in pregnant mares’ plasma and determine whether these differed between mares with healthy or compromised pregnancies. Jugular blood samples were collected between 243 and 351 days gestation from 19 healthy Thoroughbred mares and 14 mares with placental pathology, including placentitis, and other clinical problems (uterine torsion/rupture, colic, laminitis). Ten progestagens were identified using gas chromatography–mass spectrometry, of which seven increased significantly with gestational age in healthy mares while P4 was undetectable. Mares with placentitis had increased concentrations of either P5 and/or P4 and several metabolites (5α-DHP, P5ββ, ββ-diol, βα-diol, 20α-5P) suggesting increased fetal production of P5 and/or P4 and increased metabolism in the utero-placental tissues in response to chronic stress. Mares with other placental pathology had raised P4 concentrations while 5α-DHP and 3β-5P were low possibly due to reduced placental function. In mares with problems unrelated to the placenta, most progestagens were substantially lower than control values. Although progestagen profiles differed between normal and abnormal pregnancies, no clear link was demonstrated between maternal plasma concentrations of P4, 5α-DHP or any other progestagen and the maintenance of pregnancy.