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. Although not the primary aim, early pregnancy ultrasound assessments can offer the diagnostic opportunity to identify pelvic pathology. A LAMN can be identified on ultrasonography by visualisation of an adnexal mass separate to the ovary, which due to the layers of secretions has a distinctive appearance previously likened to ‘onion-skin’ or ‘whipped-cream’. Here, we describe an incidental finding of a LAMN during an early pregnancy assessment. Practitioners of early pregnancy ultrasound should be familiar with the characteristic morphology of this rare but important finding.