Fetal Circulation and Transition
On this page
Direct answer
Fetal circulation bypasses the lungs through three shunts, the ductus venosus, the foramen ovale and the ductus arteriosus, because the lungs are fluid-filled and their vascular resistance is high. At birth, the first breaths, the clamping of the cord and the rise in arterial oxygen tension convert this into adult-type circulation. The foramen ovale closes functionally at birth, while the ductus arteriosus constricts over hours to days.
What you must remember
- Umbilical vein: carries the most oxygenated blood from the placenta, and most of it is streamed onward through the ductus venosus.
- Ductus venosus: shunts blood from the umbilical vein into the inferior vena cava, and closes within the first days after birth.
- Foramen ovale: a right-to-left atrial shunt that closes functionally at birth, when left atrial pressure exceeds right atrial pressure after lung expansion.
- Ductus arteriosus: links the pulmonary artery to the aorta with right-to-left flow, and is kept open by prostaglandins while oxygen tension stays low.
- Pulmonary vascular resistance: high in fetal life, and falls sharply after the first breaths and lung expansion.
- Cord clamping: removes the low-resistance placental circuit, so systemic vascular resistance rises.
- Failed transition: persistent pulmonary hypertension of the newborn keeps pulmonary resistance high and the shunt right to left.
- Clinical clue: cyanosis that fails to improve with oxygen in a newborn points to a cardiac lesion or persistent pulmonary hypertension.
Tracing the change from fetus to newborn
Blood flow in utero is organised so that the placenta, not the lungs, oxygenates the fetus. The table below sets out each structure before and after birth, which is the form in which examiners usually test this topic.
| Structure | Fetal state | After birth |
|---|---|---|
| Ductus venosus | Open, streams umbilical blood to the inferior vena cava | Closes within days |
| Foramen ovale | Open, right-to-left atrial flow | Functional closure at birth |
| Ductus arteriosus | Open, pulmonary artery to aorta | Constricts over hours to days |
| Pulmonary resistance | High | Falls after the first breaths |
The most oxygenated blood reaches the fetal brain because the ductus venosus and the Eustachian valve direct it across the foramen ovale to the left atrium. Right-to-left flow is normal before birth, so cyanosis after birth signals a surviving shunt rather than normal physiology.
When a case describes a continuous murmur in a preterm infant, think of a persistent ductus arteriosus, which has failed to constrict. When it describes cyanosis that does not respond to oxygen, think of a shunt that has survived transition, such as a cardiac lesion or persistent pulmonary hypertension of the newborn.
Where the exam tries to catch you
Most transition questions hinge on the difference between anatomical and functional change.
- Trap: saying the foramen ovale anatomically closes at birth. Correct: it closes functionally at birth through its flap, and anatomical fusion occurs later, with some people keeping a patent foramen ovale into adulthood.
- Trap: attributing ductal constriction only to rising oxygen. Correct: falling circulating prostaglandin levels and rising oxygen tension both contribute to closure.
- Trap: confusing the right-to-left shunt of fetal life with cyanosis. Correct: the placenta oxygenates the fetus, so cyanosis after birth is abnormal and suggests a persisting shunt or pulmonary hypertension.
- Trap: thinking the ductus is open only because of its anatomy. Correct: in term infants it stays open because of prostaglandins and low oxygen tension, which is why prostaglandin E1 can reopen it in duct-dependent lesions.
Frequently asked questions
Which fetal shunt closes functionally at birth?
The foramen ovale closes functionally at birth, when the rise in left atrial pressure after lung expansion pushes its flap against the septum. Anatomical fusion occurs later, and a patent foramen ovale can persist in some adults.
Why does the ductus arteriosus close after birth?
The ductus arteriosus constricts after birth because arterial oxygen tension rises and circulating prostaglandin E2 from the placenta falls. Prostaglandin E1 given to neonates reverses this process and keeps the duct open in duct-dependent cardiac lesions.
What causes the fall in pulmonary vascular resistance at birth?
The first breaths and lung expansion raise alveolar oxygen tension and mechanically open the pulmonary vessels, which lowers pulmonary vascular resistance. Once resistance falls, blood flows through the lungs and pulmonary blood flow rises sharply.
How does the most oxygenated blood reach the fetal brain?
The ductus venosus streams highly oxygenated blood from the umbilical vein into the inferior vena cava, and the Eustachian valve and crista dividens direct it across the foramen ovale. This route supplies the brain and coronary circulation with the best-oxygenated blood in fetal life.