The 'onion skin' sign of a low-grade appendiceal mucinous neoplasm: An incidental finding during early pregnancy assessment.

A low-grade appendiceal mucinous neoplasm (LAMN) is a cystic dilatation of the appendix resulting from the accumulation of mucinous secretions caused by a luminal obstruction. Although usually benign, pseudomyxoma peritonei may occur in the event of rupture, and 10% of cases may be secondary to appendiceal cystadenocarcinoma. A LAMN is both more common and more likely to have a malignant association in women, making it an entity with which practitioners of gynaecological ultrasound should be familiar.