abnormal placenta development abnormal decidualization MP:0003845 MPheno.ontology abnormal decidual cell reaction atypical cellular and vascular changes occurring in the endometrium of the pregnant uterus just after the onset of blastocyst implantation; this process involves the proliferation and differentiation of the fibroblast-like endometrial stromal cells into large, polyploid decidual cells that eventually form the maternal component of the placenta; expected 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 defective decidualization abnormal uterine environment