Heart and Circulation in an Unborn Baby

The main difference in the blood circulation of an unborn baby versus a baby that is already born is the fact that the lungs have no function within the mother's womb. The baby is floating in the amniotic fluid and the lungs are therefore in a collapsed state.The baby gets oxygen rich blood from the mother's circulation (mother’s blood) through the placenta via the umbilical cord.

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Unique heart structures in the unborn child

The blood from the umbilical cord passes to the right upper chamber of the heart and then into the left side of the heart through a hole in the atrial septum (wall between the two top chambers of the heart) which is called the patent foramen ovale (PFO).

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Heart with patent foramen ovale (PFO)

This hole usually seals off (closes) after birth but in 30% of people can still remain open in adult life. This is a normal finding and should not cause any alarm.
The oxygen rich blood that has reached the left side of the heart in the unborn baby is then circulated through the body via the aorta to supply the brain and other organs of the baby. The right side of the heart contains oxygen poor blood and this is pumped into the pulmonary artery (artery to the lung). As the lungs are collapsed this blood does not really have an easy way forward and is diverted through the ductus arteriosus or 'duct' into the aorta which supplies the lower half of the body.

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Heart with Ductus Arteriosus

This form of circulation ensures that important organs like the brain get oxygen rich blood whilst the peripheral organs make do with oxygen poor blood. The duct usually closes within a few days after birth. In some babies, especially premature babies, the duct can remain open leading to Persistent Ductus Arteriosus or PDA which may need closure in later life.