decidua capsularis

**Definition and location:** The decidua is the progesterone-transformed endometrium of pregnancy. After interstitial implantation, the blastocyst becomes embedded in the endometrium and is covered by a thin layer of decidua called the decidua capsularis (the "capsule" over the conceptus). The three regions are: - **Decidua basalis:** lies deep to the implanted conceptus. It forms the maternal part of the placenta and is the site of placental separation at delivery (through the spongy layer). - **Decidua capsularis:** lies superficial to the conceptus, between the embryo and the uterine lumen. - **Decidua parietalis (vera):** lines the rest of the uterine cavity, away from the implantation site.

**Decidualization:** Under progesterone (initially from the corpus luteum, later from the placenta) and hCG support, endometrial stromal fibroblasts enlarge into polygonal, glycogen- and lipid-rich decidual cells. Decidual cells secrete prolactin and IGFBP-1. They help regulate trophoblast invasion and provide nutrition and immunologic tolerance. The spiral arteries are remodeled by extravillous trophoblast.

**Course through pregnancy:** - Early on, the capsularis is a thin layer. It bulges into the uterine lumen as the conceptus grows. - The chorion laeve (smooth chorion) lies under the capsularis. The villi there degenerate from reduced blood supply as the sac expands. The villi under the basalis form the chorion frondosum, which becomes the fetal part of the placenta. - By about 12-16 weeks, the sac fills the uterine cavity. The capsularis fuses with the parietalis, which obliterates the uterine cavity. The capsularis is then largely atrophic. - Fusion of the amnion and chorion follows, forming the amniochorionic membrane. Together with the adherent decidua, this forms the fetal membranes.

**Clinical correlations:** - **Placenta accreta spectrum:** Absent or deficient decidua basalis (Nitabuch layer) leads to abnormal villous adherence (accreta, increta, percreta). Risk factors include prior C-section and placenta previa. It presents with postpartum hemorrhage and a retained placenta. - **Ectopic pregnancy:** Implantation outside the uterus, most commonly in the ampullary fallopian tube, yields no true decidua capsularis. Decidual reaction can still occur in the uterine endometrium, so Arias-Stella reaction may be seen on histology. - **Ultrasound:** The "double decidual sac sign" shows two echogenic rings from the decidua capsularis and the decidua parietalis. It indicates an intrauterine pregnancy before a yolk sac is visible. - **Complete mole and abortion:** Decidual tissue in curettage specimens helps confirm an intrauterine pregnancy.

**High-yield mnemonic:** Basalis = Base (under the embryo, forms the maternal placenta). Capsularis = Capsule or Cover (over the embryo). Parietalis = Parietal (the rest of the uterine wall).