Schematic illustrating the inhibition of luteolysis in pigs. Estrogens produced by the conceptus alters the direction of prostaglandin F2α (PGF2α) secretion from an endocrine to an exocrine direction (uterine lumen). Additionally, PGF2α of uterine origin is taken up by the mesometrium and transferred into the uterus in arterial blood by a counter current system operating in the broad ligament of the uterus. Estrogen also increases prostaglandin E2 (PGE2) secretion by the pig uterus, which is hypothesized to protect the corpus luteum from the luteolytic actions of PGF2α. Estrogen also maintains luteinizing hormone receptors (LHR) in the CL and has a direct luteotropic effect. This figure was modified from Ziecik and coworkers .