Fetal circulation for tricuspid valve atresia

In fetuses with tricuspid valve atresia, there is obligate right-to-left flow of the entire systemic venous return across the foramen ovale because there is no direct communication between the right atrium and ventricle. As a result, the left ventricle, which receives all the systemic venous return, becomes the dominant ventricle. In this illustration of types 1b and 1c tricuspid valve atresia, left-to-right flow across the ventricular septal defect is the source of pulmonary blood flow.
The presence of reverse (left-to-right flow) blood flow in the ductus arteriosus (DA) suggests inadequate pulmonary outflow, which in turn indicates the need for postnatal initiation of prostaglandin therapy to maintain ductal patency. In contrast, in normal fetal circulation, the majority of the systemic venous return flows from the right atrium into the right ventricle (the dominant fetal ventricle) and through the DA into the aorta.