Decidualization is a process that occurs in early pregnancy. It is an adaption of the uterus to enable implantation of the embryo.

In preimplantation physiology, decidualization is the process by which the endometrial stroma transforms itself into a dense cellular matrix. The result is the formation of a decidua. The purpose of the decidua is to impede the invasion of trophoblasts, a feat it accomplishes not only by forming the physical barrier but also by generating cytokines promoting attachment of the trophoblast.