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Recent use of liquid chromatography mass spectrometry, measuring multiple steroid hormones simultaneously in blood, foetal and placental tissues throughout normal gestation, and in mares with experimentally induced placentitis, has advanced our current understanding of many of the unusual strategies seen during gestation and at foaling. This includes the stimulation of luteal steroidogeneisis by equine chorionic gonadotropin (eCG) from the endometrial cups, resulting in additional androgen and oestrogen secretion. Progesterone declines as the endometrial cups and eCG disappears, replaced by 5\u03b1-dihydroprogesterone (DHP), a potent equine progesterone receptor (PR) agonist, as the chorioallantoic placenta develops. Placental steroidogenesis thereafter is influenced by foetal pregnenolone and dehydroepiandrosterone secretion, providing substrate for 5\u03b1-pregnane and oestrogen synthesis, an unusual example of a \u2018foeto-placental unit\u2019. Foetal gonadal dehydroepiandrosterone fuels placental oestrone sulphate secretion, peaking at higher concentrations in mares than any other species known, declining steadily thereafter to term. Additional 5\u03b1-reduced (DHP) metabolites increase from mid-gestation to peak concentrations 3\u20135 days before foaling, declining prepartum, most likely as a result of selective loss of placental SRD5A1 (5\u03b1-reductase) expression and activity. Similar changes occur in mares with experimentally induced placentitis, which is also associated with a decreased ratio of equine PR-B:PR-A in myometrium, suggesting that progestin withdrawal is both systemic (pregnanes) and local (receptor-dependent) in mares. In addition, some steroids detected during equine pregnancy by immuno-assay are not detected by mass spectrometry, further illustrating the immense value of this technology.<\/jats:p>","DOI":"10.1530\/rep-19-0179","type":"journal-article","created":{"date-parts":[[2019,6,28]],"date-time":"2019-06-28T11:59:10Z","timestamp":1561723150000},"page":"R197-R208","update-policy":"https:\/\/doi.org\/10.1530\/crossmarkpolicy-1","source":"Crossref","is-referenced-by-count":44,"title":["Steroids in the establishment and maintenance of pregnancy and at parturition in the mare"],"prefix":"10.1093","volume":"158","author":[{"given":"Alan J","family":"Conley","sequence":"first","affiliation":[{"name":"Department of Population Health & Reproduction , School of Veterinary Medicine, University of California, Davis, California, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Barry A","family":"Ball","sequence":"additional","affiliation":[{"name":"Gluck Equine Research Center , Department of Veterinary Science, University of Kentucky, Lexington, Kentucky, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"286","published-online":{"date-parts":[[2019,12,1]]},"reference":[{"key":"2026010712200193800_bib1","doi-asserted-by":"crossref","first-page":"127","DOI":"10.1530\/jrf.0.1110127","article-title":"Immunolocalization of 3 beta-hydroxysteroid dehydrogenase, cytochrome P450 17 alpha-hydroxylase\/17,20-lyase and cytochrome P450 aromatase in the equine corpus luteum of dioestrus and early pregnancy","author":"Albrecht","year":"1997","journal-title":"Journal of Reproduction and Fertility"},{"key":"2026010712200193800_bib2","doi-asserted-by":"publisher","first-page":"821","DOI":"10.1095\/biolreprod56.4.821","article-title":"Differential transcription of steroidogenic enzymes in the equine primary corpus luteum during diestrus and early pregnancy","author":"Albrecht","year":"1997","journal-title":"Biology of Reproduction"},{"key":"2026010712200193800_bib3","doi-asserted-by":"publisher","first-page":"551","DOI":"10.1016\/S0093-691X(01)00425-3","article-title":"Expression of 3beta-hydroxysteroid dehydrogenase, cytochrome P450 17alpha-hydroxylase\/17,20-lyase and cytochrome P450 aromatase enzymes in corpora lutea of diestrous and early pregnant mares","author":"Albrecht","year":"2001","journal-title":"Theriogenology"},{"key":"2026010712200193800_bib4","doi-asserted-by":"publisher","first-page":"5803","DOI":"10.1210\/jc.2004-0479","article-title":"Regulation of placental vascular endothelial growth\/permeability factor expression and angiogenesis by estrogen during early baboon pregnancy","author":"Albrecht","year":"2004","journal-title":"Journal of Clinical Endocrinology and Metabolism"},{"key":"2026010712200193800_bib5","doi-asserted-by":"publisher","first-page":"593","DOI":"10.1677\/joe.0.0430593","article-title":"The immunological measurement of pregnant mare serum gonadotrophin","author":"Allen","year":"1969","journal-title":"Journal of Endocrinology"},{"key":"2026010712200193800_bib6","doi-asserted-by":"publisher","first-page":"237","DOI":"10.1677\/joe.0.1720237","article-title":"The influence of maternal size on placental, fetal and postnatal growth in the horse. 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