What is the primary reason blood is propelled through the foramen ovale in fetal circulation?

The primary reason blood is propelled through the foramen ovale in fetal circulation is related to the hemodynamics present in the fetal heart and circulation. During fetal development, the lungs are not yet inflated, which results in a high-pressure environment in the right atrium. With the lungs being deflated, there is minimal blood flow to the pulmonary circulation, causing the right atrial pressure to exceed that of the left atrium. This pressure gradient promotes the flow of oxygenated blood from the right atrium to the left atrium through the foramen ovale.

This shunting allows the oxygen-rich blood, which is received from the placenta via the umbilical vein, to bypass the non-functioning lungs and be delivered directly to the systemic circulation, supporting the developing fetus.