The uterine decidua is the transformed inner lining of the uterus (endometrium) that changes in preparation for and during pregnancy. It creates a receptive environment for the blastocyst, supporting the establishment and maintenance of a healthy pregnancy. It is a dynamic component of the maternal-fetal interface, supporting the developing embryo and fetus.
Formation of the Uterine Decidua
The transformation of the endometrium into the decidua is a process called decidualization. This process is primarily influenced by hormonal signals, specifically progesterone, which is produced after ovulation. Estrogen also contributes to this preparatory phase, enhancing the endometrium’s sensitivity to progesterone.
During decidualization, the typically spindle-shaped endometrial stromal cells differentiate into plump, secretory decidual cells. These cells produce various proteins and hormones, including prolactin and insulin-like growth factor-binding protein-1 (IGFBP-1).
The extracellular matrix surrounding these cells also changes, becoming rich in fibronectin and laminin, resembling an epithelial-like structure. Vascularity and vascular permeability also increase within the decidualizing endometrium, ensuring a richer blood supply. This transformation happens monthly in anticipation of a fertilized egg, and if pregnancy does not occur, this specialized lining is shed during menstruation.
Essential Functions During Pregnancy
The uterine decidua performs several functions throughout pregnancy, supporting the developing embryo and fetus. It provides the initial site for blastocyst implantation, allowing the embryo to burrow into the uterine wall and connect with the maternal blood supply. This secure attachment is important for pregnancy continuation.
Before the placenta is fully formed and functional, the decidua nourishes the early embryo. It provides a nutrient-rich and vascularized environment, delivering essential substances to the developing embryo. This early support is important for the embryo’s initial growth.
The decidua also helps to modulate the maternal immune response. The developing fetus is semi-allogeneic, meaning it possesses paternal genetic material that could be recognized as foreign by the mother’s immune system. The decidua helps prevent the rejection of the fetus by producing immunosuppressive factors and creating a protective barrier.
The decidua protects the uterine wall from excessive invasion by trophoblast cells. It regulates the depth of trophoblast invasion, ensuring the placenta attaches appropriately without penetrating too deeply into the uterine muscle. This controlled invasion is important for maintaining uterine integrity.
The decidua also contributes to hormone production during pregnancy. It produces various hormones, such as progesterone and human chorionic gonadotropin (hCG), which are important for maintaining the pregnancy. These hormones contribute to the overall hormonal environment necessary for fetal growth and development.
What Happens When the Decidua Is Abnormal
Abnormalities in the formation or function of the decidua can lead to various pregnancy complications. One such condition is placenta accreta spectrum disorders, where the placenta abnormally attaches to the uterine wall. In these cases, defects in the decidua can allow placental trophoblast cells to invade too deeply, sometimes into the uterine muscle or even beyond.
Impaired decidualization can also be associated with recurrent pregnancy loss. If the decidual lining does not transform properly, it may not adequately support embryo implantation or early development, leading to early pregnancy failure. A dysfunctional decidua may not provide the necessary environment or immune modulation for a successful pregnancy.
In an ectopic pregnancy, where the fertilized egg implants outside the uterus, the decidual reaction can still occur in the uterine lining, even without an embryo present. This highlights the hormonal influence on decidualization, as the uterine lining still prepares for pregnancy despite the abnormal implantation site. However, this decidual change cannot support the misplaced embryo.
Decidual dysfunction has also been linked to preeclampsia, a serious pregnancy complication characterized by high blood pressure and organ damage. Issues with decidual development and its interaction with invading trophoblast cells are thought to contribute to the abnormal placental development seen in preeclampsia. These decidual issues can affect the remodeling of maternal blood vessels, impacting blood flow to the placenta.