Normal development. (A) day 5. Note the uterine lumen between the two decidualized endometrial layers (arrows). (B) day 6. The myometrium has a lower echogenicity (arrowheads) compared to the decidualized endometrium. The high echodensity spot indicates the embryo. (C) Histological section of implantation site shown in (B). (D) Day 7. The embryonic cavity and the embryo proper are visible. Decidua capsularis and basalis are differentiated. (E) Day 8. Embryo with amnion and allantois. (F) Day 9. The hyperechogenic decidua capsularis is stretched out at the antimesometrial side and merges into the decidua basalis at the mesometrial side. The embryonic brain ventricles and the neural canal are visible. (G) Histological section of implantation site shown in (F). Reichert’s membrane and yolk sac membrane are only visible in the histologic section. In vivo, these membranes are stretched out against the decidua and the placenta. (H) Day 10. The aortic arches and the mesencephalon are depicted. (I) Histology of the same embryo as in (H). (J) Day 11. The placenta displays hyperechogenic calcification deposits (arrowheads) at the fetomaternal boundary. (K) Histological section of placenta of same embryo as shown in (J). The giant trophoblast is disappearing. (L) Histology of same embryo as in (J and K) outlining the transition zone between decidua capsularis, decidua basalis and new uterine lumen. AA - Aortic arches; Al – Allantois; Am - Amnion; DB - Decidua basalis; DC - Decidua capsularis; Dec – Decidua; EC – Embryonic cavity; ECC – Excocoelomic cavity; Em- Embryo; EPC – Ectoplacental cone; FE – Fetal erythrocytes; He – Heart; La – Labyrinth; Mes – Mesencephalon; Ms – Mesometrium; Myo – Myometrium; NC – Neural canal; nUL – new uterine lumen; oUL – old uterine lumen; RM – Reichert's membrane; Pc – Pericardium; Pl – Placenta; St – Syncytiotrophoblast; UC – Umbilical connection; UL – Uterine lumen; UV – Umbilical vessel; VV – Vitelline vessel; YS – Yolk sac.