Fetal Circulation: Anatomy and Transition at Birth

Fetal circulation differs fundamentally from adult circulation because the placenta, not the lungs, serves as the organ of gas exchange. The fetal circulatory system includes unique shunts that bypass the nonfunctional fetal lungs.

Overview of Fetal Circulation

Key Differences from Adult Circulation

Fetal Circulatory Structures

The Placenta

The placenta is the fetal organ of gas exchange, nutrient delivery, and waste removal.

Structure:

• Fetal side: Chorionic villi with fetal capillaries
• Maternal side: Intervillous space with maternal blood
• Separation: Two cell layers (syncytiotrophoblast and fetal endothelium)

Umbilical cord vessels:

• One umbilical vein: Carries oxygenated blood from placenta to fetus
• Two umbilical arteries: Carry deoxygenated blood from fetus to placenta

The Three Fetal Shunts

Ductus Venosus

Location: Connects the umbilical vein to the inferior vena cava

Structure:

• Narrow channel through the liver
• Sphincter-like smooth muscle
• Regulates blood flow through the liver

Function:

• 40-60% of umbilical venous blood bypasses the liver
• Remaining blood perfuses the hepatic sinusoids
• Delivers oxygenated blood to the IVC

Closure:

• Functional: Minutes after birth (loss of umbilical flow)
• Anatomic: 1-2 weeks after birth
• Remnant: Ligamentum venosum (on the liver)

Foramen Ovale

Location: Interatrial septum

Structure:

• Flap valve created by the septum primum and septum secundum
• Inferior vena cava directs blood toward the foramen ovale
• Valve opens from right to left

Function:

• Directs oxygenated blood from IVC to left atrium
• Bypasses the right ventricle and pulmonary circulation
• Accounts for approximately 30-40% of combined cardiac output

Direction of shunting:

• Fetal: Right-to-left (IVC blood to LA)
• Atrial pressures: Right atrial > Left atrial (fetal)
• Adult: Left-to-right or closed

Closure:

• Functional: Minutes after birth (increased LA pressure)
• Anatomic: 3-12 months (septal fusion)
• Remnant: Fossa ovalis

Ductus Arteriosus

Location: Between the pulmonary trunk and the descending aorta

Structure:

• Muscular artery (thick wall)
• High sensitivity to oxygen and prostaglandins
• 10-15 mm long, 5-10 mm diameter

Function:

• Diverts blood from the pulmonary trunk to the aorta
• Bypasses the high-resistance pulmonary circulation
• Accounts for approximately 50-60% of right ventricular output

Direction of shunting:

• Fetal: Right-to-left (pulmonary trunk to aorta)
• Due to high pulmonary vascular resistance

Closure:

• Functional: 10-15 hours after birth (smooth muscle contraction)
• Anatomic: 2-3 weeks (fibrotic obliteration)
• Remnant: Ligamentum arteriosum

Mechanism of closure:

• Increased oxygen tension (constriction)
• Decreased prostaglandin E2 (PGE2 levels fall after birth)
• Endothelin-1 release
• Bradykinin release

Blood Flow in Fetal Circulation

Oxygen Saturation Levels

Flow Distribution

Directional Flow Patterns

Flow from the Placenta to the Heart

1. Umbilical vein carries oxygenated blood from the placenta
2. Ductus venosus: ~50% of umbilical venous blood bypasses the liver
3. Portal sinus: Remaining blood enters the liver
4. Inferior vena cava: Receives blood from the ductus venosus and the lower body
5. Crista dividens: The eustachian valve directs IVC blood toward the foramen ovale

Preferential Streaming

The fetal circulation uses preferential streaming to deliver the most oxygenated blood to the brain and heart:

Well-oxygenated blood path:

• Umbilical vein → Ductus venosus → IVC → Foramen ovale → LA → LV → Aorta (pre-ductal) → Carotid arteries → Brain

Less-oxygenated blood path:

• SVC → RA → RV → Pulmonary trunk → Ductus arteriosus → Descending aorta → Umbilical arteries → Placenta

Changes at Birth

Sequence of Events

Phase 1: Delivery and first breath

1. Chest compression during vaginal delivery
2. Expansion of the lungs (first breath)
3. Dramatic decrease in pulmonary vascular resistance
4. Increased pulmonary blood flow (10× increase)

Phase 2: Shunt closure

1. Ductus venosus: Loss of placental flow → passive closure
2. Foramen ovale: Increased LA pressure (from increased pulmonary venous return) + decreased RA pressure (loss of placental flow) → functional closure
3. Ductus arteriosus: Increased PO₂ → smooth muscle constriction → functional closure within hours

Phase 3: Permanent closure

1. Anatomic remodeling and fibrosis over weeks to months
2. Ligamentous remnants remain

Cardiorespiratory Transition at Birth

Persistent Fetal Circulation (PPHN)

Persistent pulmonary hypertension of the newborn occurs when the pulmonary vascular resistance remains high after birth, causing right-to-left shunting across the foramen ovale or ductus arteriosus.

Causes:

• Meconium aspiration syndrome
• Congenital diaphragmatic hernia
• Sepsis
• Birth asphyxia
• Pulmonary hypoplasia

Clinical Significance of Fetal Remnants