In mice and women, terminal differentiation of uterine natural killer (uNK) cells commences during endometrial decidualization. Both proliferation and interferon (IFN)-γ are induced. Uterine NK cell precursors appear to home from secondary lymphoid organs to decidualizing uteri and localize mesometrially to the central decidua basalis, the site of maternal arterial modification at Gestation Days (gd) 9.5–10. In mice, genetic absence of uNK cells results in absence of pregnancy-induced spiral artery modification. Administration of IFN-γ to uNK-negative pregnant females induces arterial modifications without fetal loss. In this study, we investigated the roles of cytokines, known in other tissues to differentiate and activate NK cells, in induction of IFN-γ production in normal mouse implantation sites. Fecundity evaluation, implantation site morphometry, and IFN-γ quantification in interleukin (IL)-12p40^0/0, IL-18^0/0, dual IL-12p40^0/0/IL-18^0/0 and congenic strains revealed the importance of both IL-12 and IL-18 in the induction of spiral artery modification and IFN-γ synthesis. Immediately after implantation, IL-18 was localized transiently to decidual cells, but by gd8, IL-18 was produced solely by uNK cells, suggesting that early uNK cells are activated by stroma and lymphocyte-derived signals maintain later uNK cell activation. Mesometrial tissue of C57Bl/6J mice was examined by reverse transcription polymerase chain reaction assay in virgin, early postimplantation, and midgestation females for expression of the heterodimeric cytokines IL-23 (composed of IL-12p40 and a novel α chain), IL-27 (composed of two IL-12-related chains) and IL-27R. No expression was detected in virgin uteri. The four genes were induced by gd6, and uNK cells isolated from midgestation transcribed IL-23α and IL-27R. This study advances the understanding of uNK cell activation during normal pregnancy.