Arias-Stella reaction
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