Breast cancer is the most common malignant tumor in women of reproductive age, and fertility preservation counseling is now an integral part of the initial management of these patients. This article reports the case of a 33-year-old woman diagnosed with breast cancer and referred for oncofertility counseling before her treatment. Despite a previous negative cancer workup, a transvaginal ultrasound scan, performed for antral follicle count as part of the initial ovarian reserve assessment, revealed a synchronous ovarian adenocarcinoma. A BRCA1 mutation was confirmed weeks later. This report highlights the role of transvaginal ultrasound in the initial evaluation and reviews the risks associated with fertility preservation in breast cancer patients.