Decidualization is a characteristic of the endometrium of the pregnant uterus. It is a response of maternal cells to progesterone. Decidualization may be used to describe any change due to progesterone. These changes include the eosinophilic proliferation around arterioles after ovulation or progesterone action on endometrium which increases glandular epithelial secretion, stimulates glycogen accumulation in stromal cell cytoplasm, and promotes stromal vascularity (spiral arterioles) and edema . The sum of all the changes in the endometrium is significant, thus justifying a new term. The process is decidualization, the endometrium is now called decidua and it is ready for the implantation of the embryo.

After ovulation, in mammals, the endometrial lining becomes transformed into a secretory lining in preparation of accepting the embryo. Without implantation, the secretory lining will be absorbed (estrous cycle) or shed (menstrual cycle).

With implantation the lining now termed decidua

Decidua

Decidua is the term for the uterine lining during a pregnancy, which forms the maternal part of the placenta. It is formed under the influence of progesterone and forms highly-characteristic cells.-Etymology:...

evolves further during the pregnancy.
The decidua is shed during the third phase of the parturition process.