Embryology · Fetal period, placenta and twins
Fetal circulation and the changes at birth !
- Process
Before birth, blood picks up oxygen in the placenta, not the lungs. Three shortcuts steer blood past the liver and lungs, and they close at birth.
Updated 2026-11-01
See it in 3D: Ligamentum arteriosum · Interatrial septum · Portal vein · Inferior vena cava · Right atrium · Left atrium · Pulmonary trunk · Falciform ligament · Internal iliac arteries
What it is
In the fetus the placenta does the lungs' job, so the circulation uses three shunts: the ductus venosus, the foramen ovale and the ductus arteriosus. With the first breath and cord clamping the pressures change and the shunts close, first functionally and later anatomically. If they fail to close, heart defects such as patent ductus arteriosus result.
Key facts
- The foramen ovale closes functionally at birth, when the septum primum is pressed against the septum secundum; anatomical fusion takes months.
- The foramen ovale never fuses in about one person in four (patent foramen ovale).
- The ductus arteriosus closes functionally within hours to days as its smooth muscle contracts; anatomical obliteration takes weeks to months.
- Fetal blood copes with low oxygen thanks to fetal haemoglobin, which binds oxygen more tightly.
Step by step
- The umbilical vein brings oxygenated blood from the placenta towards the fetal liver.
- Much of that blood goes through the ductus venosus straight into the inferior vena cava, bypassing the liver.
- In the right atrium, the edge of the septum secundum steers the inferior caval stream through the foramen ovale into the left atrium.
- From the left ventricle it enters the ascending aorta: the coronaries, head and arms get the best-oxygenated blood.
- Blood from the superior vena cava goes to the right ventricle and the pulmonary trunk.
- Because the collapsed lungs resist flow, nearly all of it crosses the ductus arteriosus into the descending aorta.
- The descending aorta supplies the lower body, and the umbilical arteries, branches of the internal iliacs, return blood to the placenta.
- At birth, cord clamping and the first breath reverse the pressures: the foramen ovale, ductus arteriosus and ductus venosus close.
Fetal structure and its adult remnant
| Fetal structure | Adult remnant |
|---|---|
| Umbilical vein | Round ligament of the liver (ligamentum teres) |
| Ductus venosus | Ligamentum venosum |
| Foramen ovale | Fossa ovalis |
| Ductus arteriosus | Ligamentum arteriosum |
| Umbilical arteries (distal part) | Medial umbilical ligaments |
| Umbilical arteries (proximal part) | Superior vesical arteries |
| Allantois (urachus) | Median umbilical ligament |
Clinical notes
- Patent ductus arteriosus: commoner in preterm infants and in congenital rubella; continuous «machinery» murmur.
- In duct-dependent heart defects (for example pulmonary atresia or transposition), prostaglandins are used to keep the duct open.
- Persistent pulmonary hypertension of the newborn: pulmonary resistance fails to fall and blood keeps crossing the foramen and the duct.
- A patent foramen ovale can allow a paradoxical embolism in adults.
Test yourself
Think of the answer, then tap the question to see it.
Which fetal vessel carries the most highly oxygenated blood?
The umbilical vein, which brings freshly oxygenated blood from the placenta.
Why does most blood in the fetal pulmonary trunk pass through the ductus arteriosus?
Because the fluid-filled fetal lungs, with constricted vessels, have very high resistance; blood takes the shortcut to the descending aorta.
What closes the foramen ovale at birth?
Falling pulmonary resistance raises return to the left atrium; its pressure exceeds the right and presses the septum primum against the septum secundum.
What does the ductus venosus become in the adult?
The ligamentum venosum, on the visceral surface of the liver.
What keeps the ductus arteriosus open?
Prostaglandins, mainly placental PGE2; at birth they fall and the rise in oxygen constricts the duct.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
- Ligamentum arteriosum
- Interatrial septum
- Portal vein
- Inferior vena cava
- Right atrium
- Left atrium
- Pulmonary trunk
- Falciform ligament
- Internal iliac arteries
Related
- Umbilical cord: two arteries, one vein
- Placenta: structure, circulation and functions
- Atrial septation: septum primum, septum secundum and foramen ovale
- Aortic arch arteries and their derivatives
- Venous system: vitelline, umbilical and cardinal veins
- Patent ductus arteriosus
- Atrial septal defect and patent foramen ovale
- Embryonic heart structures and their adult derivatives (the table to know)
Related conditions
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Patent foramen ovale: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Patent ductus arteriosus: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 13: Cardiovascular System (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.