Arias-Stella reaction in an adenomyomatous polyp of the uterus

We describe a rare case of an Arias-Stella reaction in an adenomyomatous polyp of the endometrium found in the first trimester of pregnancy. A 33-year-old Japanese woman, gravida 1, para 1, presented for an initial prenatal examination at 6-week gestation; ultrasound revealed a myoma-like solid mass in the cervical canal. An abnormal Papanicolaou's smear (highly suspicious of adenocarcinoma) prompted the resection of this pedunculated polyp. Histologically, the musculature located in the center of the polyp was covered by the endometrium; numerous glands within both the endometrium and the musculature exhibited an Arias-Stella reaction. This explains the result of Papanicolaou's smear, because subsequent smears - both intrapartum and postpartum - were negative. To our knowledge, this is the first reported incident of an Arias-Stella reaction in an adenomyomatous polyp. This could be one of the diagnostic pitfalls in Papanicolaou's smears taken during pregnancy.