Schematic diagrams depicting cross-sections of implantation sites on days 4, 7, and 13 of pregnancy. (A) On day 4, the luminal epithelium closes on an implanting blastocyst. The mural trophectoderm, which is distant from the inner cell mass, contacts the epithelium at the antimesometrial side (AM). The decidual response also starts from the antimesometrial side (not shown). M, mesometrial side. (B) On day 7, the embryo is much larger, and its ectoplacental cone has penetrated the mesometrial decidua, which is enriched with blood vessels. Differentiated decidual cells now take up most of the implantation site (IS). Note that decidualization does not occur at inter-implantation sites. (C) On day 13, the placenta has developed, and the decidua has regressed to thin layers around the placenta and embryo, known as decidua basalis and decidua capsularis, respectively.