Fetal circulation is the pattern of blood flow that supplies a developing fetus with oxygen and nutrients through the placenta rather than through breathing air. Three specialized pathways—the ductus venosus, foramen ovale, and ductus arteriosus—redirect blood around much of the liver and lungs before birth.
This arrangement differs from circulation after birth because the fetal lungs are not yet responsible for oxygen exchange. Understanding these temporary structures connects cardiovascular development with human anatomy and explains how blood flow changes when a newborn begins breathing.
What Is Fetal Circulation?
Fetal circulation refers to the movement of blood between the fetal heart, developing organs, and placenta during pregnancy. Since the fetus does not breathe air, oxygen is supplied through the placenta instead of the lungs.
The fetal heart pumps blood through both the body and pulmonary arteries, but only a relatively small portion reaches the lungs. Most blood is redirected through temporary connections called shunts.
These adaptations operate alongside other changes described in Pregnancy Physiology Overview. They also differ from the liver’s ordinary portal blood flow, explained in Portal Circulation in Digestive Absorption.
How the Placenta Supplies Oxygen and Nutrients
The placenta is an exchange organ connecting maternal and fetal circulation. Oxygen and nutrients move from maternal blood toward fetal blood, while carbon dioxide and certain waste products move in the opposite direction.
Maternal and fetal blood normally remain within separate circulatory spaces rather than mixing freely. Exchange occurs across specialized placental tissues.
The umbilical cord contains three major blood vessels:
- One umbilical vein: Carries relatively oxygen-rich blood from the placenta toward the fetus.
- Two umbilical arteries: Carry relatively oxygen-poor blood from the fetus back to the placenta.
This is the reverse of the usual association between arteries and oxygen-rich blood. Arteries are defined by carrying blood away from the heart, while veins carry blood toward it.
The Three Shunts in Fetal Circulation
Three temporary connections redirect blood through the fetal cardiovascular system. Each serves a different anatomical function.
Ductus Venosus: Partially Bypassing the Liver
The ductus venosus connects the umbilical venous pathway with the inferior vena cava. It allows a portion of oxygen-rich placental blood to bypass the liver’s smaller vessels and reach the heart more directly.
Not all umbilical venous blood bypasses the liver. Some enters the hepatic circulation, supporting liver development and metabolism.
Foramen Ovale: Redirecting Blood Between the Atria
The foramen ovale is an opening between the right and left atria of the fetal heart. It allows a substantial portion of blood entering the right atrium from the inferior vena cava to pass into the left atrium.
From there, blood moves through the left ventricle into the ascending aorta. This pathway helps supply the heart muscle, brain, and upper body with relatively oxygen-rich blood.
Ductus Arteriosus: Bypassing Most Pulmonary Circulation
The ductus arteriosus connects the pulmonary artery to the descending aorta. Because resistance in the fetal lung circulation is high, much of the right ventricle’s output passes through this connection instead of traveling through the lungs.
It sends blood directly into systemic circulation, allowing it to supply the lower body and return toward the placenta.
Step-by-Step Blood Flow in the Fetus
Following the main blood pathways makes fetal circulation easier to understand. The same basic pumping action described in How the Heart Pumps Blood operates before birth, but the shunts change where blood travels.
- Oxygen-rich blood travels from the placenta through the umbilical vein.
- Some blood passes through the liver, while another portion uses the ductus venosus to reach the inferior vena cava.
- Blood enters the right atrium, where much of the relatively oxygen-rich stream passes through the foramen ovale.
- The left atrium and left ventricle pump this blood into the ascending aorta, supplying the heart and upper body.
- Blood returning from the upper body enters the right atrium through the superior vena cava and primarily flows into the right ventricle.
- The right ventricle pumps blood into the pulmonary artery. Most passes through the ductus arteriosus into the descending aorta.
- Blood supplies the lower body, and a portion returns to the placenta through the two umbilical arteries.
These pathways are not completely separate. Blood mixes at several locations, but preferential streaming helps deliver blood with relatively higher oxygen content to the heart and brain.
Why Fetal Blood Mostly Bypasses the Lungs
Before birth, the lungs are filled with fluid and do not exchange oxygen with inhaled air. Their blood vessels have relatively high resistance, limiting pulmonary blood flow.
The placenta provides the oxygen exchange needed for fetal development. The foramen ovale and ductus arteriosus allow much of the cardiac output to avoid the high-resistance pulmonary circulation.
Some blood still reaches the lungs, supplying developing lung tissue. Therefore, fetal circulation reduces pulmonary blood flow rather than stopping it entirely.
An Example of Oxygen Distribution Before Birth
Fetal blood normally has a lower oxygen saturation than blood in a healthy adult’s systemic arteries. For example, oxygen saturation in the umbilical vein is often approximately 70–80%, although values vary with gestational age and physiological conditions.
Despite this lower saturation, fetal tissues receive oxygen through adaptations including fetal hemoglobin, which has a higher affinity for oxygen than adult hemoglobin.
Preferential blood flow also helps direct relatively oxygen-rich blood toward the developing brain and heart. The result is an effective oxygen delivery system suited to life before birth.
What Changes in Circulation at Birth?
Birth produces major changes in blood pressure, oxygen availability, and vascular resistance. The first effective breaths expand the lungs, increase oxygen levels, and sharply reduce resistance in the pulmonary blood vessels.
As placental circulation ends, systemic vascular resistance rises. More blood travels through the lungs, and the temporary fetal shunts begin closing.
| Fetal Structure | Change After Birth | Usual Adult Remnant |
|---|---|---|
| Foramen ovale | Functionally closes as left atrial pressure exceeds right atrial pressure | Fossa ovalis |
| Ductus arteriosus | Constricts as oxygen rises and prostaglandin signaling changes | Ligamentum arteriosum |
| Ductus venosus | Closes after umbilical venous flow stops | Ligamentum venosum |
| Umbilical vein | Closes after placental circulation ends | Ligamentum teres hepatis |
Functional closure can occur quickly, but permanent anatomical closure often takes longer. The timing differs among structures and between newborns.
Fetal vs. Postnatal Circulation
The main difference between fetal and postnatal circulation is the organ responsible for oxygen exchange and the pathways blood follows to reach it.
| Feature | Before Birth | After Birth |
|---|---|---|
| Oxygen source | Placenta | Lungs |
| Pulmonary blood flow | Relatively low | Greatly increased |
| Foramen ovale | Allows right-to-left atrial flow | Normally functionally closed |
| Ductus arteriosus | Diverts blood into the aorta | Normally closes |
| Placental circulation | Active | Discontinued |
When Fetal Circulatory Connections Remain Open
Some fetal connections persist after birth or close later than expected. Their clinical effects depend on the structure, blood flow, and other cardiovascular conditions.
- Patent ductus arteriosus (PDA): The ductus arteriosus remains open, potentially allowing abnormal blood flow between the aorta and pulmonary artery.
- Patent foramen ovale (PFO): The flap between the atria does not completely seal. Many people have no symptoms.
- Persistent pulmonary hypertension of the newborn: Pulmonary vascular resistance remains abnormally high, potentially allowing right-to-left shunting and reducing oxygen delivery.
Some congenital heart conditions depend on an open ductus arteriosus to maintain adequate blood flow until treatment is available. For this reason, whether a fetal connection should close depends on the newborn’s specific cardiovascular anatomy.
Frequently Asked Questions
Does a fetus receive oxygen through its lungs?
No. Before birth, oxygen is transferred from maternal blood across the placenta into fetal blood. The fetal lungs receive some blood flow but are not responsible for exchanging oxygen with inhaled air.
What are the three main fetal shunts?
The ductus venosus partially bypasses the liver, the foramen ovale directs blood between the atria, and the ductus arteriosus redirects much of the right ventricle’s output away from the lungs.
Do all fetal shunts close immediately after birth?
No. Functional closure may begin within minutes or hours as circulation changes, but permanent anatomical closure often takes days, weeks, or longer. Some connections, particularly the foramen ovale, may remain incompletely sealed.
Fetal circulation uses the placenta for oxygen exchange and three specialized shunts to redirect blood before the lungs become functional. At birth, breathing and the loss of placental circulation change pressure relationships, increasing pulmonary blood flow and initiating shunt closure. These changes establish the circulation needed for life outside the uterus.
This article is for general education and is not medical advice. Talk to a qualified health professional about any medical concern.
Sources: Guyton and Hall Textbook of Medical Physiology; The Developing Human: Clinically Oriented Embryology; American Heart Association.
