Mesenchymal hamartoma of the chest wall (MHCW) is a rare condition. Previously, surgical resection has been advocated with considerable post-operative morbidity. Evidence for conservative management is lacking because the natural history of MHCW is unknown. We present serial measurements of an antenatally detected MHCW (8 antenatal ultrasounds and 2 postnatal computed tomographic scans). The study demonstrates that the relative tumor size peaked at birth and then decreased postnatally. Based on this evidence, we believe that MHCW can be managed conservatively in an asymptomatic patient.