decidua

Decidua is the modified lining of the uterus (the endometrium) that forms during pregnancy under the influence of progesterone. It lines the uterus and provides the maternal component of the placenta, supporting implantation and fetal development. The decidua is eventually shed after delivery as part of the placental site and is involved in the postpartum process known as lochia.

The decidua is divided into three regions: decidua basalis, decidua capsularis, and decidua parietalis. The decidua basalis lies beneath the implanted conceptus and gives rise to most of the maternal portion of the placenta. The decidua capsularis covers the portion of the conceptus facing the uterine cavity. The decidua parietalis (also called the decidua vera) lines the remainder of the uterine cavity. As pregnancy progresses, the capsularis and parietalis come into closer contact and may fuse as the uterus enlarges.

Decidualization refers to the transformation of endometrial stromal cells into large, glycogen-rich, lipid-rich decidual cells. This process is driven by progesterone and supports implantation by providing a rich, nutrient-containing environment and by modulating maternal immune responses to promote tolerance of the fetus. Decidual tissue also interacts with the invading trophoblasts to regulate invasion and placental development.

In the postpartum period, the decidua is shed along with the placenta at birth. The placental bed undergoes extensive remodeling and vascular regression, and remnants of decidual tissue are broken down and discharged in lochia. Abnormalities of the decidua can be involved in obstetric complications, but normally it functions as a crucial maternal-fetal interface during pregnancy.