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.