Fetal aortic isthmus growth and morphology in late gestation.

Objective: To establish normal values for fetal aortic isthmus diameter in late gestation and to identify any changes in aortic isthmus dimensions and morphology in pathological conditions.
Methods: In this prospective study, the fetal aortic isthmus was evaluated in 110 low-risk pregnant women at between 30 and 40 weeks of gestation and 42 pregnant women who were at high risk for congenital heart defects. From coronal echocardiographic images of the connection between the aorta and ductus arteriosus, the internal diameter of the aorta was measured at the middle of the isthmus, at the point of the isthmus just proximal to the entry of the ductus arteriosus and at the descending aorta below the entry of the ductus arteriosus.
Results: Correlation coefficients for the diameter of each aortic segment when related to gestational age varied from r = 0.60 to r = 0.80 (P < 0.001 for each), and growth curves were derived from the third and 97th percentiles about each linear regression analysis. The mean and the third percentile for the ratio of the isthmus just proximal to the entry of the ductus arteriosus to the middle of the isthmus were 1 and 0.81. In one fetus of the high-risk patients, a contraductal shelf and the accompanying area of tubular isthmic hypoplasia were suspected and a diagnosis of coarctation of the aorta was subsequently confirmed after birth. In two fetuses with growth restriction and one fetus with intestinal atresia, the isthmus diameters were below the third percentile but the ratios of the isthmus end to the middle of the aortic isthmus were all normal and no cardiac anomalies were detected after birth.
Conclusions: We could establish normal values for aortic isthmus diameters in late gestation from a coronal view and identify even minimal changes in aortic isthmus dimensions and morphology in pathological conditions.