Longitudinal Magnetic Resonance Imaging Study of Hemodynamic Changes in the Third Trimester in Fetuses With Major Congenital Heart Defects and Healthy Fetuses.

To assess hemodynamic changes in the third trimester by magnetic resonance imaging (MRI) in fetuses with major congenital heart defects (CHD). Twenty fetuses with CHD and 23 healthy controls had consecutive MRI at 28, 32, and 38 weeks of gestation. Blood flow in the ascending aorta, main pulmonary artery, and umbilical vein was measured using phase contrast MRI and indexed to estimated fetal weight. Mixed effects models assessed weight-indexed changes over time and between groups. CHD cases were further classified into left-sided defects, right-sided defects, or transposed great arteries for subgroup analysis. Combined ventricular output (CVO) decreased by 1.7% per week (95% CI 0.7%-2.7%) from 28 to 38 weeks in both fetuses with CHD and controls, with no difference between groups (p = 0.3). Umbilical vein (UV) flow decreased by 2.5% per week (95% CI 1.5%-3.5%) with no difference between groups (p = 0.7). CVO was correlated with UV flow (r = 0.6, 95% CI 0.4-0.7). The aortic-to-pulmonary flow ratio remained constant in all subgroups. Left- and right-sided defects were associated with increased flow across the contralateral side. CVO decreased similarly in fetuses with CHD and controls during the third trimester, associated with reduced UV flow, reflecting a more efficient placenta toward term.