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Abstract 38 39 Conceptus-uterine communication is established during trophoblastic elongation 40 when the conceptus synthesizes and releases estrogen, the maternal recognition signal in 41 the pig. Interleukin-1β (IL-1β) is a differentially expressed gene during rapid 42 trophoblastic elongation in the pig. The current investigation determined conceptus and 43 endometrial changes in gene expression for IL-1β, IL-1 receptor antagonist (IL-1Rant), 44 IL-1 receptor type 1 (IL-1RT1) and IL-1 receptor accessory protein (IL-1RAP) in 45 developing peri- and post-implantation conceptuses, as well as uterine endometrium 46 collected from cyclic and pregnant gilts. Conceptus IL-1β gene expression was enhanced 47 during the period of rapid trophoblastic elongation compared to earlier spherical 48 conceptuses, followed by a dramatic decrease in post-elongated day 15 conceptuses. IL-49 1RT1 and IL-1RAP gene expression was greater in day 12 and 15 filamentous 50 conceptuses compared to earlier morphologies while IL-1Rant gene expression was 51 unchanged by conceptus development. The uterine lumenal content of IL-1β increased 5