decidua parietalis

**Definition and the three decidual regions** Decidua is the hormonally transformed (progesterone-driven) functional layer of endometrium in pregnancy. Its stromal cells enlarge into glycogen- and lipid-rich decidual cells. It has three regions: - **Decidua basalis**: underlies the implanted blastocyst. It forms the maternal part of the placenta and is the site of placental attachment. - **Decidua capsularis**: overlies (encapsulates) the conceptus on the luminal side. - **Decidua parietalis (vera)**: lines the rest of the uterine cavity, away from the implantation site.

**Development and fusion** - As the gestational sac grows, the decidua capsularis bulges into the uterine lumen. By about 12–16 weeks it contacts and fuses with the decidua parietalis on the opposite wall. This obliterates the uterine cavity. - The decidua capsularis degenerates as it is stretched and loses its blood supply. The chorion laeve (smooth chorion) and amnion then come to abut the decidua parietalis. - The fetal membranes (amnion, chorion laeve) and the adherent decidua parietalis make up the structures that rupture at membrane rupture.

**Function and clinical significance** - **Decidualization** is driven by progesterone and by signals from the implanting embryo. It provides nutrition, immunologic tolerance, and regulation of trophoblast invasion. - **Ectopic pregnancy**: Decidual reaction still occurs in the endometrium without an intrauterine pregnancy. Sloughed decidua may be passed as a decidual cast. The Arias-Stella reaction (hypersecretory endometrial glands with atypical, enlarged nuclei) can occur with ectopic and intrauterine pregnancy and is not malignant. - **Placenta accreta spectrum**: Defective or absent decidua basalis (e.g., over a prior cesarean scar or in placenta previa) allows chorionic villi to attach to or invade the myometrium. The basalis, not the parietalis, is the layer that fails here. - **Postpartum**: The decidua is shed with the placenta and membranes as the lochia. The basal layer of the endometrium remains and regenerates the endometrium. Retained membranes or decidua can cause postpartum hemorrhage or endometritis. - **Imaging**: On early ultrasound, the "double decidual sign" (decidua parietalis and decidua capsularis seen as two concentric echogenic rings around the gestational sac) suggests a true intrauterine gestational sac. This is used to distinguish it from a pseudogestational sac of ectopic pregnancy. The sign is nonspecific and less reliable than seeing a yolk sac. - **Immunology**: Decidual NK cells, macrophages, and Tregs in the decidua help maintain maternal tolerance of the semi-allogeneic fetus.

**High-yield mnemonic**: Basalis = Base of placenta. Capsularis = Capsule over embryo. Parietalis = Parietal wall (everything else).