Morphometric analysis of the great vessels in early fetal life.

Pathological examination of the heart and great vessels was performed in 61 specimens obtained after surgical termination of pregnancy for psychosocial indications at 9-18 weeks of gestation. The aorta and pulmonary trunk were identified and external diameters were measured at the level of, and distal to the aortic valve and pulmonary valve, the level of the aortic isthmus and thoracic aorta, and the proximal and distal ductus arteriosus. All eight vessel diameters increased linearly with gestational age and the ratio of the diameter of the aortic isthmus to that of the aortic valve or the distal ductus arteriosus also increased with gestation. Early pregnancy is characterized by rapid growth of the fetal head and this may well be the consequence of a preferential distribution of left ventricular output in favour of the head due to relative narrowing of the aortic isthmus at this gestation.