The first figure shows a cross-section through the heart at this stage. While in the womb, blood is oxygenated within the placenta - a rich nexus of blood vessels where the fetal circulation exchanges oxygen and receives nutrients from the maternal circulation. Exchange occurs in the placenta as the lungs do not function until birth. Thus the heart must shunt blood away from the lungs. It accomplishes this in a variety of ways.
Blood entering the right atrium is shunted to the left atrium across the foramen ovale.
In utero , pressures in the right side of the heart are greater than in the left side. Once the lungs fill with air, however, the situation reverses. Pressures on the left side are then greater than on the right. Blood making it into the right ventricle can readily cross into the left ventricle as the interventricular septum has not yet formed to separate the right ventricle from the left ventricle. Blood entering the Pulmonary arteries from the right ventricle can cross directly into the aorta via a connection called the ductus arteriosis .