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Fetal MRI

Fetal Anatomy and Circulation

The fetal circulation differs greatly from the postnatal circulation. The site of gas exchange in the fetus is the placenta as opposed to the lungs post-natally. In the normal postnatal circulation there is no mixing of oxygen rich pulmonary venous blood with less rich systemic venous blood whereas the fetal circulation contains several sites where blood with different oxygen concentrations mix.

In the normal fetal circulation, well-oxygenated blood returning from the placenta through the umbilical vein is split between the two sides of the liver and the ductus venosus. Up to 50 per cent of the umbilical vein blood is thought to bypass the liver and is directed into the thoracic portion of the inferior vena cava via the ductus venosus. The remainder of the umbilical vein blood mixes with the portal venous return, although portal venous blood is preferentially directed to the right lobe of the liver compared with the left. Blood from the hepatic veins rejoins blood from the ductus venosus and enters the thoracic inferior vena cava. In the fetal lamb, the thoracic inferior vena cava contains two streams of blood, a stream of well oxygenated blood emerging from the left liver and ductus venosus, and another stream of less oxygenated blood from the right liver and abdominal portion of the inferior vena cava.

Blood from the thoracic portion of the inferior vena cava enters the right atrium where it is joined by blood from the superior vena cava. Here, the Eustachian valve and lower portion of the atrial septum move together to direct blood through the foramen ovale or through the tricuspid valve. In fetal lambs there is streaming of the more oxygenated blood from the ductus venosus and left liver across the foramen ovale, resulting in an oxygen saturation of ~65 per cent in the left atrium, while slightly less oxygenated blood (~55 per cent) enters the right ventricle through the tricuspid valve. Blood entering the left atrium via the foramen ovale combines with the less oxygenated blood from the pulmonary veins and enters the left ventrical through the mitral valve.  From the left ventricle, blood is ejected into the ascending aorta to feed the coronary, cerebral and upper body circulations. Although some blood from the right ventricle enters the pulmonary circulation, most is directed via the ductus arteriosus into the descending aorta, supplying the placenta, via the umbilical arteries, and the lower body.

An important aspect of the normal fetal circulation is that the left and right heart act in parallel. Fetal blood that is delivered to the left liver, left heart, brain and upper body has higher oxygen content than blood delivered to the right liver, right heart, lungs, lower body and placenta.

R. Uflacker, Atlas of Vascular Anatomy: An Angiographic Approach, 2nd ed., vol. 132, no. 11. Am Med Assoc, 2007.
A. M. Rudolph, Congenital Diseases of the Heart: Clinical-Physiological Considerations, 3rd ed. Wiley Blackwell, 2009.
Sørensen A, Holm D, Pedersen M, Tietze A, Stausbøl-Grøn B, Duus L, Uldbjerg N. The left-right difference in fetal liver oxygenation during hypoxia, as estimated by BOLD MRI in a fetal sheep model, Ultrasound in Obstetrics & Gynecology, 2011.