Circulation

Before birth, blood from the right side of the heart passes straight to the left side through the foramen ovale and the ductus arteriosus.
After birth these holes close and blood is then diverted through the lungs which, of course, do not oxygenate the blood before birth.
Failure of these holes to close, or the existence of other holes, causes many of the congenital heart defects that require corrective surgery or intervention.