Fetal circulation is the unique blood flow system that keeps a baby alive before birth, routing oxygen-rich blood from the placenta while bypassing the lungs and most of the liver. Unlike circulation after birth, where the lungs handle oxygen exchange, a fetus relies entirely on the placenta to receive oxygen and nutrients from the mother’s blood. To make this work, the fetal heart and blood vessels include three temporary shortcuts, called shunts, that redirect blood away from organs that aren’t yet functional.
How Blood Gets From the Placenta to the Fetus
The umbilical cord contains three vessels: one umbilical vein and two umbilical arteries. The naming here is counterintuitive. The single umbilical vein carries freshly oxygenated, nutrient-rich blood from the placenta to the baby. The two umbilical arteries carry used, oxygen-depleted blood back to the placenta for replenishment.
Fetal blood oxygen levels are much lower than what you’d see in a breathing person. Fetal arterial oxygen saturation averages around 58%, compared to 95% or higher in a newborn breathing air. That saturation continues to drop as the oxygenated blood from the umbilical vein mixes with deoxygenated blood on its way through the fetal body. The system is designed to deliver the most oxygen-rich blood to the organs that need it most: the brain, the heart, and the upper body.
Three Shunts That Reroute Blood Flow
The fetal circulatory system relies on three bypass routes that don’t exist in a functioning adult heart. Each one diverts blood away from an organ that isn’t needed yet.
The Ductus Venosus: Bypassing the Liver
When the umbilical vein enters the baby’s body at the belly button, it branches. A small portion of blood flows into the liver through a channel called the portal sinus, giving the liver the oxygen and nutrients it needs to develop. But the majority of blood takes a shortcut called the ductus venosus, which channels it directly into the inferior vena cava, the large vein that feeds into the heart. Without this bypass, all the oxygenated blood would have to filter through the liver first, losing oxygen content along the way before ever reaching the brain or heart.
The Foramen Ovale: Bypassing the Lungs (Part 1)
Once blood reaches the heart through the inferior vena cava, it enters the right atrium. In a breathing person, blood in the right atrium gets pumped to the lungs to pick up oxygen. But a fetus’s lungs are collapsed, filled with amniotic fluid, and not doing any gas exchange. Sending blood there would be wasted effort.
Instead, most of the blood in the right atrium flows through a small opening in the wall between the two upper chambers of the heart, called the foramen ovale. This lets oxygenated blood pass directly from the right atrium into the left atrium, skipping the lungs entirely. From the left atrium, blood moves into the left ventricle and out through the aorta, the body’s main artery. This is the route that delivers the highest-oxygen blood to the brain, the heart muscle, and the arms.
The Ductus Arteriosus: Bypassing the Lungs (Part 2)
Not all blood in the right atrium crosses through the foramen ovale. Some follows the normal route into the right ventricle and gets pumped toward the lungs through the pulmonary artery. But even here, the fetal body has a backup plan. A vessel called the ductus arteriosus connects the pulmonary artery directly to the descending aorta, letting most of that blood skip the lungs and flow to the lower body instead. Less than 10% of the blood leaving the right ventricle actually reaches the lung tissue. The rest joins the systemic circulation through this shortcut. From the descending aorta, blood travels to the lower body and eventually returns to the placenta through the two umbilical arteries to pick up fresh oxygen.
Why Fetal Blood Prioritizes the Brain
The design of fetal circulation creates a deliberate hierarchy. The most oxygen-rich blood coming from the placenta is fast-tracked through the ductus venosus, across the foramen ovale, and up through the aorta to reach the brain, heart, and upper body first. By the time blood reaches the lower body and returns to the placenta, it has progressively lost oxygen content. This oxygen gradient ensures the developing brain receives the best supply available, even though overall fetal oxygen levels are low by postnatal standards.
What Happens at the First Breath
The entire fetal circulatory system transforms within minutes of birth. The trigger is the baby’s first breath, which typically happens within about 10 seconds of delivery and often sounds like a gasp. That breath inflates the lungs for the first time, and a cascade of changes follows.
As air fills the lungs, blood flow resistance in the pulmonary vessels drops dramatically. Blood suddenly rushes into the lungs to begin picking up oxygen the normal way. At the same time, clamping the umbilical cord cuts off the low-resistance placental circuit, which raises the overall pressure in the baby’s systemic blood vessels. This pressure shift is critical: it raises the pressure in the left atrium above the pressure in the right atrium, which pushes a flap of tissue over the foramen ovale and closes it.
The ductus arteriosus responds to the sudden increase in oxygen. Within about 10 minutes of breathing, the smooth muscle in its wall constricts, reversing the blood flow direction and eventually sealing the vessel shut. The ductus venosus also closes once blood stops flowing through the umbilical vein. Over the following days and weeks, all three shunts permanently seal with scar tissue, and the baby’s circulation operates the same way an adult’s does.
When the Transition Doesn’t Go Smoothly
Sometimes the fetal-to-newborn transition fails to complete properly. When the blood vessels in the lungs remain constricted after birth, blood continues to bypass the lungs through the fetal shunts as if the baby were still in the womb. This condition is called persistent pulmonary hypertension of the newborn, sometimes referred to as persistent fetal circulation.
Several factors can contribute. Infections, aspiration of meconium (the baby’s first stool) during delivery, and underdeveloped lungs all increase the risk. Certain maternal factors also play a role, including diabetes, obesity, pre-eclampsia, and smoking. Structural heart defects can produce similar circulation problems.
The hallmark sign is low blood oxygen despite the baby breathing. Doctors typically confirm the diagnosis with an echocardiogram, which can show the direction of blood flow through the fetal shunts and measure pressure in the heart chambers. Treatment focuses on reducing the resistance in the lung blood vessels so the baby’s circulation can complete its normal transition.
In most healthy newborns, however, the shift from fetal to adult-type circulation happens seamlessly. The three temporary shunts that kept the fetus alive close on schedule, and the lungs take over the job the placenta held for nine months.