During week 7, tissue separating the left and right atria grows and an opening between the atria called the foramen ovale appears. This opening lets blood pass from one atrium to the other, allowing it to bypass the lungs. At birth, the opening closes. The ventricle walls are very muscular. Blood is pumped from the left ventricle to the body and brain, and from the right ventricle to the lungs.
Heart valves develop at the same time as the chambers.
These valves fill and empty the heart. Heart sounds and heart murmurs are caused by blood passing through these valves. Your baby gets oxygen from you. Blood from your baby flows to the placenta through the umbilical cord. In the placenta, oxygen and nutrients are transported from your blood to the fetal blood.