Background and Purpose Aneurysmal bone cysts (ABCs) rarely involve the cranium and have seldom been reported in pregnancy. Clinical Presentation We describe a case of a 28-year-old woman who presented at 37 weeks of gestation with 3 months of gradually worsening vision, 10 months of proptosis, and restricted ocular motility on the left. Brain imaging revealed a multicystic enhancing mass measuring 5.9 × 5.3 × 3.7 cm, centered on the cribriform plate on the left, extending into the anterior cranial fossa superiorly as well as the left nasal cavity, maxillary, sphenoid, and frontal sinuses. Her clinical course is described in detail; 3-month postoperative imaging demonstrated no residual mass. Conclusion A literature review revealed five previous cases of ABCs associated with pregnancy. We report a rare case of a giant ABC of fibrous dysplasia involving the paranasal sinuses and anterior cranial fossa. We postulate on the possible influence of pregnancy on the clinical course.