A 31-year-old female was in the 13th week of pregnancy when an abdominal ultrasound examination revealed a large retroperitoneal tumour. Magnetic resonance imaging was carried out and the imaging described a 10-cm mass in diameter extending from the right kidney. Given that the patient was in her first trimester and that there was a suspicion of malignancy, further surgical exploration of the tumour was warranted. During the operation, the tumour was removed, but nephrectomy was not necessary. Histologic analysis of the resected tumour showed a mucinous cystic adenoma, and no signs of malignancy were present. Following the surgery, the pregnancy was otherwise uneventful and further complications did not occur. This case illustrates that surgery is recommended in patients with a retroperitoneal tumour early during a pregnancy, when a malignancy cannot be excluded.