Implantation, physiology of placentation

Many pregnancies fail to implant and thus are not usually recognized clinically. Implantation, trophoblast development, and placentation are crucial in the establishment and development of normal pregnancy. Abnormalities of these events can lead to pregnancy complications known as the great obstetrical syndromes: preeclampsia, intrauterine growth restriction, fetal demise, and recurrent pregnancy loss (RPL). The definition of RPL varies; however, most include two or more failed clinical pregnancies as documented by ultrasonography or histopathologic examination. Preimplantation, vascularization, invasion, and oxidative stress are among the main players in the tuning and function of these events and are the leading elements in positive outcomes of the pregnancy. The preparatory process for pregnancy starts with the postovulatory surge in circulating progesterone levels. This surge inhibits estrogen-dependent proliferation of the uterine epithelium and induces secretory transformation of the uterine glands. Subsequently, the luminal epithelium expresses an evolutionarily conserved repertoire of molecules essential for stable interaction and adherence of a blastocyst, thus enabling implantation. New data demonstrate that coordinated migration and invasiveness of decidualizing endometrial stromal cells in response to embryonic and trophoblast signals are key to successful implantation.