Challenges in Diagnosing Uterine Leiomyosarcoma in Pregnancy: A Case Report and Lessons Learned.

Uterine leiomyosarcoma (uLMS) is a rare, highly malignant neoplasm arising from the smooth muscle of the uterus and is exceedingly rare in pregnancy. We present a unique case highlighting the risk of misdiagnosis of uLMS as the more common condition of benign degenerating leiomyoma in pregnancy. Our case involves a 35-year-old, primiparous woman, where on a third-trimester scan (30 weeks), a high index of suspicion was raised for uLMS in a pre-existing benign leiomyoma. However, subsequent MRI assessments and multidisciplinary team (MDT) discussions inaccurately attributed it to the degeneration of a fibroid. uLMS was only diagnosed on histopathological examination when she underwent myomectomy for the presumed fibroid eight months postpartum. This caused a significant delay in the definitive surgical management and contributed to substantial iatrogenic upstaging of the disease (i.e., widespread metastasis of parametrial extension, pelvic lymphadenopathy, and pulmonary metastases), ultimately necessitating palliative second-line chemotherapy. Our case report aims to enhance clinicians' understanding of the atypical clinical and radiological features of uLMS with improved detection and management.