decidual

Decidual refers to the specialized form of the endometrium that appears during pregnancy. It results from decidualization, a process in which endometrial stromal cells transform in response to the hormonal environment, notably progesterone after ovulation. The decidua forms the maternal part of the placenta and supports embryo implantation and placental development. It is classically divided into three regions: decidua basalis (beneath the implanted conceptus, forming the maternal portion of the placenta), decidua capsularis (overlying the embryo within the uterus), and decidua parietalis (the remainder of the endometrium that lines the uterine cavity).

The decidual tissue consists of enlarged, secretory stromal cells known as decidual cells, along with immune cells such as natural killer cells and macrophages, and a supporting extracellular matrix. Decidual cells secrete cytokines, growth factors, and other mediators that promote embryo development, regulate trophoblastic invasion, and remodel maternal spiral arteries to optimize blood flow to the growing fetus. The decidua also contributes to maternal immune tolerance by shaping the local immune environment to protect the semi‑allogeneic fetus.

Clinically, the decidua persists throughout pregnancy and typically regresses after delivery. Abnormal decidualization or decidual deficiency can be associated with impaired placentation and pregnancy complications. Additionally, for individuals with endometriosis, ectopic endometrial tissue can undergo decidualization in response to pregnancy hormones, a phenomenon used to understand tissue remodeling and symptom changes during pregnancy.