Cross-talk between the embryo and mother is necessary for the establishment of successful pregnancy. In the mouse, cervical stimuli during mating induce prolactin secretion from the pituitary and formation of the corpus luteum (CL) of pregnancy. Placental lactogen supports the maintenance of pregnancy after implantation. In the human, the embryo secretes chorionic gonadotropin (hCG), which has lenbinizing hormone (LH) activity and maintains pregnancy before the luteal-placental shift occurs. In cattle and sheep, the mechanism underlying the establishment of pregnancy is unique. Their embryos secrete interferon (IFN)-τ, which is related to type I IFNs. IFN-τ prevents the secretion of prostaglandin F_2α(PGF_2α) from the uterine endometrium by suppressing the expression of estrogen and oxytocin receptor. In the pig, estrogen secreted from the conceptus is a factor for maternal recognition of pregnancy. Estrogen leads to a shift in the direction of PGF_2α secretion from endocrine to exocrine and inhibits the regression of CL. Thus, each species has specific mechanisms for the establishment of pregnancy.