Rectus abdominis endometriosis presenting as a menstruation-linked abdominal mass

Rectus abdominis endometriosis (RAE) is a rare form of extra-pelvic endometriosis, often associated with prior surgical or gynecological procedures. We report a case of a woman in her early 30 s who presented with an eight-month history of a painful, cyclical mass in the left lower abdomen. Her history included a cesarean section, medical abortion, and failed IVF. Imaging identified a 2.3 × 1.5 cm hypoechoic nodule within the rectus abdominis muscle. Surgical excision confirmed endometriotic tissue, and the patient had an uneventful recovery. Histopathology revealed ectopic endometrial glands and stroma within the rectus muscle, consistent with RAE. This case highlights the potential for RAE to occur years after gynecological interventions and in atypical locations, suggesting alternative pathogenic mechanisms such as vascular spread or immune dysregulation. In this patient, delayed presentation and lesion location away from the cesarean scar raise the possibility of multifactorial pathogenesis involving both iatrogenic and non-iatrogenic factors, including hormonal stimulation from IVF. Complete surgical excision led to the resolution of symptoms. Clinicians should consider RAE in patients presenting with cyclical abdominal wall masses, even in the absence of recent surgery.