BACKGROUND: Small left heart structures are observed in fetuses with left-sided congenital diaphragmatic hernia (CDH). Fetoscopic tracheal occlusion (FETO) in mid-gestation promotes lung growth in fetuses with CDH, however cardiac effects of FETO are poorly described. We studied the effects of FETO on cardiac structure size at birth, hypothesizing that left heart structures would be larger in neonates who had undergone fetal intervention. METHODS/RESULTS: We performed retrospective measurements of atrioventricular and semilunar valve and pulmonary artery diameters, ventricular lengths, left ventricular end-diastolic volume indexed (LVEDVi) to body surface area. 35 patients were studied (9 FETO, 26 controls). All fetuses had liver herniation and a lung-to-head ratio <1 at fetal presentation. At birth the intervention group had larger LVEDVi (16.8 vs. 12.76 ml/m(2), p < 0.05), LV length Z-score (-2.05 vs. -4, p < 0.01), LV:RV length ratio (1.43 vs. 1.04, p < 0.05), LPA diameter Z-score (+1.71 vs. -1.04, p < 0.05), and better growth of aortic valve (-2.18 FETO, -3.3 controls, p < 0.01). There was a trend toward higher LV output in the FETO group. CONCLUSIONS: Left heart structures and LPA were larger postnatally in patients with CDH who underwent FETO than in those who did not. Hemodynamic alterations are introduced with tracheal occlusion that are associated with alterations in ventricular loading and may influence growth.