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We report the case of a woman with metastatic alveolar soft part sarcoma (ASPS) who experienced pregnancy-associated disease progression followed by unexpected postpartum regression. Pulmonary lesions increased in size after a miscarriage and during a subsequent pregnancy, prompting consideration for systemic treatment and potential enrolment in a clinical trial evaluating a tyrosine kinase inhibitor in combination with an immune checkpoint inhibitor. However, baseline staging CT performed a few weeks postpartum unexpectedly demonstrated marked spontaneous regression. Because trial enrolment required Response Evaluation Criteria in Solid Tumors-defined radiographic progression, she was deemed ineligible, and no systemic therapy was initiated. Surveillance imaging over the following 2 years confirmed sustained regression and stability of residual disease in the absence of treatment. This case highlights the potential influence of pregnancy-to-postpartum physiology on tumour behaviour through hormonal, immune and angiogenic mechanisms. It also emphasises the importance of short-interval reassessment after delivery before initiation of systemic therapy in reproductive-age patients with metastatic ASPS.