Growth of the great vessels in the normal human fetus and in the fetus with cardiac defects.

Background: There is a paucity of quantitative anatomic data regarding human great vessel development that could be useful as a reference for fetal echocardiographers who must distinguish abnormal from normal cardiac development at early stages.

Methods And Results: To determine normal growth patterns, we plotted the diameters of the aortic and pulmonary valves, ductus arteriosus, aortic isthmus, and descending aorta in 274 autopsy specimens from nonselected spontaneous abortuses of normal karyotype. There was a linear increase in the diameters of these structures within the developmental period studied (10-26 weeks). A relative narrowing of the aorta at the isthmus compared with the aortic valve and descending aorta probably indicates that the majority of fetal left heart output goes to the developing heart and brain. In contrast to previous studies of late gestation and neonatal animals, however, we found that the diameter of the aortic isthmus was larger than that of the ductus arteriosus, suggesting substantial isthmic blood flow in these midtrimester fetuses. Among nineteen other hearts with diverse defects, both of two hearts with a narrow isthmus had an enlarged ductus arteriosus and one heart with pulmonary atresia/intact septum had a narrow ductus and increased aortic valve diameter.

Conclusions: During midgestation, the normal heart may have substantial aortic isthmic blood flow that diminishes due to rerouting in late gestation when increased requirements of the fetal brain and other organs prevail. Although fetal shunts may explain some vessel abnormalities, the majority of cardiac defects in this study were not associated with abnormal growth of the great vessels within this developmental age range.