Fetal Circulation and Changes at Birth :Best article 2025

Overview of Fetal Circulation

Fetal circulation is uniquely designed to allow oxygenated blood from the placenta to bypass non-functional fetal lungs and provide oxygen and nutrients to the developing organs. Key structures and pathways include:

1. Placenta: The site of oxygen and nutrient exchange between maternal and fetal blood.
2. Umbilical Vein: Carries oxygenated blood from the placenta to the fetus.
3. Ductus Venosus: Shunts a portion of oxygenated blood from the umbilical vein directly into the inferior vena cava, bypassing the liver.
4. Foramen Ovale: An opening between the right and left atria, allowing oxygenated blood to flow from the right atrium to the left atrium, bypassing the lungs.
5. Ductus Arteriosus: Connects the pulmonary artery to the descending aorta, bypassing the pulmonary circulation.
6. Umbilical Arteries: Carry deoxygenated blood from the fetus back to the placenta.

Blood Flow Pattern in Fetal Circulation

1. Oxygenated blood enters via the umbilical vein, with some bypassing the liver through the ductus venosus to the inferior vena cava.
2. From the inferior vena cava, blood flows into the right atrium.

• Most of it shunts through the foramen ovale into the left atrium, then to the left ventricle and systemic circulation (head and heart).
• The remaining blood enters the right ventricle and pulmonary artery.

1. Blood in the pulmonary artery is diverted to the aorta via the ductus arteriosus, bypassing the lungs.
2. Deoxygenated blood returns to the placenta through the umbilical arteries.

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Changes in Circulation at Birth

At birth, major physiological changes occur due to the transition from placental to pulmonary gas exchange. These changes include:

1. Lung Expansion:

• First breaths expand the lungs, reducing pulmonary vascular resistance and increasing pulmonary blood flow.
• Oxygenation causes pulmonary arterioles to dilate.

1. Closure of the Foramen Ovale:

• Increased left atrial pressure (due to increased pulmonary venous return) and decreased right atrial pressure (due to loss of placental circulation) result in functional closure of the foramen ovale.

1. Closure of the Ductus Arteriosus:

• Elevated oxygen tension and reduced levels of prostaglandins stimulate the ductus arteriosus to close functionally within 10–15 hours and anatomically within 2–3 weeks.

1. Closure of the Ductus Venosus:

• The ductus venosus closes within days to weeks after birth as blood flow from the umbilical vein ceases.

1. Umbilical Vessels Obliteration:

• The umbilical vein becomes the ligamentum teres (round ligament of the liver).
• The umbilical arteries form the medial umbilical ligaments.

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Clinical Correlation

1. Persistent Fetal Circulation: Also called Persistent Pulmonary Hypertension of the Newborn (PPHN), occurs when the pulmonary vascular resistance remains elevated, causing right-to-left shunting through the foramen ovale or ductus arteriosus.
2. Patent Ductus Arteriosus (PDA): Failure of the ductus arteriosus to close leads to left-to-right shunting, causing pulmonary overcirculation.
3. Atrial Septal Defects (ASD): Result from failure of the foramen ovale to close properly.

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