Arias-Stella reaction, also Arias-Stella phenomenon, is a benign change in the endometrium associated with the presence of chorionic tissue. Arias-Stella reaction is due to progesterone primarily. Cytologically, it looks like a malignancy and, historically, it was diagnosed as endometrial cancer. Significance It is significant only because it can be misdiagnosed as a cancer. It may be seen in a completely normal pregnancy. Diagnosis It is characterized by nuclear enlargement and may also have any of the following: an irregular nuclear membrane, granular chromatin, centronuclear vacuolization, and pseudonuclear inclusions. Five subtypes are recognized: Minimal atypia. Early secretory pattern. Secretory or hypersecretory pattern. Regenerative, proliferative or nonsecretory pattern. Monstrous cell pattern. History It was first described by Javier Arias Stella, a Peruvian pathologist, in 1954. See also Choriocarcinoma Chorioangioma Herpes Nuclear atypia Nuclear pleomorphism Molar ectopic References Bibliography Textbook of Obstetrics by D.C. Dutta Page no. 180. Mammal female reproductive system