Here's a breakdown:
* Before pregnancy: The endometrium is a thin layer of tissue, ready to receive a fertilized egg.
* During pregnancy: The endometrium thickens considerably and becomes highly vascularized. It transforms into the decidua, providing a nourishing environment for the developing embryo/fetus.
* After pregnancy: The decidua sheds after childbirth, resulting in postpartum bleeding. This is known as lochia.
Decidua can be further subdivided into three layers:
* Decidua basalis: The layer closest to the myometrium (the muscular wall of the uterus), forming the maternal part of the placenta.
* Decidua capsularis: The layer surrounding the developing embryo, eventually thinning and disappearing as the placenta develops.
* Decidua parietalis: The remaining lining of the uterus, not directly involved in placental development.
Therefore, "decidua" is a specific term referring to the modified endometrial lining during pregnancy, crucial for supporting the developing embryo/fetus and the formation of the placenta.