Evolutionary studies on different classes of vertebrates could help clarify the role of cytokines in acceptance of the embryo by the maternal tissues. This review focuses on the cytokine interleukin-1 (IL-1) and reports on its presence in the female reproductive tract of species with different reproductive strategies, that is, viviparity, oviparity, and ovuliparity. Unlike oviparity and viviparity, ovuliparity does not involve any contact between paternal-derived fetal antigens and maternal tissues, because eggs are released unfertilized in the external environment. Therefore, we consider ovuliparity a natural negative control for mechanisms of materno-fetal immunotolerance. The goal of this review is to discuss the role of the IL-1 system in the acquisition of the ability to retain the embryo in the female genital tract during the transition from ovuliparity to viviparity.