The involvement of estradiol at the time of implantation in placental mammals

The local presence of estradiol seems to characterize the implantation period in placental mammals. Even though estradiol produced by the ovaries occasionally plays a large role, its local production by the embryo and/or the endometrium seems to be general. The steroid activity of the embryo and the endometrium shows wide qualitative and quantitative variations, depending on the species. The role of these local estrogens at the time of implantation is practically unknown.