Micrograph showing<unk>  of the endometrium due to exogenous progesterone (oral contraceptive pill). H&E stain.<unk>  is a process that results in significant changes to cells of the endometrium in preparation for, and during, pregnancy. This includes morphological and functional changes (the decidual reaction) to endometrial stromal cells (ESCs), the presence of decidual white blood cells (leukocytes), and vascular changes to maternal arteries. The sum of these changes results in the endometrium changing into a structure called the decidua. In humans, the decidua is shed during the third phase of birth.<unk>  plays an important role in promoting placenta formation between a mother and her fetus by mediating the invasiveness of trophoblast cells. It also triggers the production of cellular and molecular factors that result in structural changes, or remodeling, of maternal spiral arteries.<unk>  is required in some mammalian species where embryo implantation and trophoblast cell invasion of the endometrium occurs, also known as hemochorial placentation. This allows maternal blood to come into direct contact with the fetal chorion, a membrane between the fetal and maternal tissues, and allows for nutrient and gas exchange. However,<unk> -like reactions have also been observed in some species that don't display hemochorial placentation. In humans,<unk>  occurs after ovulation during the menstrual cycle. After implantation of the embryo, the decidua further develops to mediate the process of placentation. In the event no embryo is implanted, the decidualized endometrial lining is shed or, as is the case with species that follow the estrous cycle, absorbed. In menstruating species,<unk>  is spontaneous and occurs as a result of maternal hormones. In non-menstruating species,<unk>  is non-spontaneous, meaning it only happens after there are external signals from an implanted embryo.
Decidualization
Foreclosure 
Avolition
Decapitation
Informal coercion
Violence interruption
Co-option 
Mitigation