[Hormonal requirements for blastocyst implantation]

In the early stages of pregnancy, embryo development and preparation of the endometrium must be synchronized in a very precise manner, in order to let blastocyst establishment. This state results from the interactions of steroid hormones, progesterone and/or estrogens. The hormonal requirements for embryo implantation vary considerably among different mammals. Systemic progesterone intervention is common to the various species; however, the requirement for ovarian estrogen seems not to be. In this paper, certain new aspects of the hormonal requirements for blastocyst implantation are reviewed.