A 39-year-old G3P3 female presented with abrupt onset dyspnoea of one month duration. She was markedly symptomatic when lying supine and resorted to prone sleeping. Chest X-ray reported as cardiomegaly. Transthoracic echocardiography was unremarkable twice. Computed tomography chest showed a dilated pulmonary artery. Transesophageal echocardiography identified a 12-mm ostium secundum atrial septal defect with mild pulmonary hypertension. The defect was closed with a cocoon device and rendered her symptom free. This case highlights the importance of timely organization of transesophageal echocardiography when transthoracic echocardiography is negative. It also illustrates marked dyspnoea could be a presentation of undiagnosed atrial septal defect with mild pulmonary hypertension.