There may be a narrowing at some point which means the pumping
chamber behind has to work harder. There may be a hole between the two
sides of the heart allowing blood to flow from one side to the other (a “hole-in-the-heart”) or part of the heart may be missing. Sometimes the
way the heart is connected to the veins or arteries is inappropriate, so that
the organs get blood of the wrong colour, and if blue blood goes round the
body the complexion is bluish (“blue baby”). More than one of these
abnormalities may be present, and, of course, there are different degrees of
severity.
Rarely, in a heart that is normally formed, one or other of the pumping
chambers may be weak.