The heart begins its story as two tiny tubes that form about three weeks after conception of the fetus.  The two tube fuse together to form a single chamber smaller than the size of the period at the end of this sentence!  As early as the fourth week of pregnancy, the tiny heart begins to beat.  Blood begins washing back and forth through this tiny structure as the walls swell, twist and toughen.  In an amazing symphony of movement and growth, the tiny tube forms an "S" shape as walls fuse and chambers form.  Slowly the back and forth movement of blood becomes one-way circulation.  By about 8 weeks of gestation, the tiny heart has the basic appearance of the mature heart, with upper and lower chambers.  Walls form in the middle fo these two chambers forming the two atria above and the two ventricles below.  Valves form at the two exits from the heart, the pulmonary artery (which takes blood to the heart) and the aorta (which takes blood to the body).  Another set of valves form to prevent blood from flowing backwards from the ventricles to the atria (the mitral and tricuspid valves). 

The fetal lungs

Although the fetal heart begins pumping just 4 weeks into the pregnancy, the lungs remain quiet until after birth.  Air doesn't enter the lungs until the baby takes the first breath.  Only a small amount of blood is pumped through the fetal lungs.  After birth, all the baby's blood will pass through the lungs.

The fetal circulation

The fetal blood receives oxygen and nutrients from the mother's blood in the placenta.  The fetal blood and mother's blood remain separated by blood vessel walls.  Oxygen and nutrients pass through the vessel walls, while blood cells are not allowed to pass.  The oxygen-enriched blood from the placenta travels to the fetus by way of the inferior vena cava which takes blood to the right atrium.  Here some blood passes through a hole in the wall (foramen ovale) to the left atrium, bypassing the lungs.  The rest of the blood passes to the right ventricle and out to the pulmonary artery.  Since the lungs have yet to be inflated with oxygen-rich air, much of the blood bypasses the lungs through a temporary blood passage called the ductus arteriosus.  Fetal blood is delivered to all the body parts of the fetus and then is returned to the placenta by way of the umbilical arteries that leave the fetus at the site of the future belly button.

Birth - the transition from fetal to baby circulation

The birth of a child is amazing beyond words!  Trust me, I am a parent!  With removal of the placenta, the baby's circulatory system must make an immediate transition to provide it's own nutrients and oxygen.  As air fills the baby's lungs, the pulmonary arteries also fill with blood until all the baby's blood is traveling through the lungs.  The connection that bypasses the lungs (ductus arteriosus) and the umbilical vessels close soon after birth.  With time, these vessels wither away to become ligaments.  The connection between the two atria (foramen ovale) may take several months to close completely. 

Abnormal development of the heart

For unknown reasons, the development of some fetal hearts is disrupted or incomplete.  This can lead to congenital heart defects.  If the walls between the chambers don't form completely, a child may be left with an abnomal "hole in the heart."  An abnormal connection between atria is called an atrial septal defect (ASD).  An abnormal connection between ventricles is called a venritcular septal defect (VSD).  More complex defects can result from incomplete formation of heart walls or abnormal separation of heart vessels.

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