Asymptomatic female in her early 30s presented for her singleton nuchal translucency ultrasound at 12.4 weeks of gestational age. No fetal abnormalities were seen. A large maternal left adnexal mass was identified, prompting a transvaginal ultrasound. The findings were felt to be consistent with an endometrioma, albeit with a 1 cm solid nodule. At this point, the diagnosis was considered to likely represent a decidualised endometrioma; however, warranting careful follow-up. The 20-week anatomy scan demonstrated an enlarging solid nodule that now contained vascularity within the left adnexal mass prompting an MRI. These features were more suggestive of malignancy, although a decidualised endometrioma, which can demonstrate intrapartum growth and vascularity, remained in the differential. Intrapartum sonographic surveillance documented progressive growth. The lesion was uneventfully removed at the time of delivery. Pathology showed clear cell carcinoma within an endometrioma.