Antenatal diagnosis of right heart enlargement has a wide spectrum of differential diagnosis from maternal, placental and fetal causes, and outcomes of all are not known. Coarctation of the aorta is in the differential diagnosis of right heart enlargement. In our study, we focused to measure multiple cardiac dimensions in fetuses with right heart enlargement to identify the fetus with coarctation of the aorta utilizing echocardiographic measurements. Ten cardiovascular dimensions were measured from fetal studies between 20- and 34-week gestation, and six were measured on postnatal echocardiograms. Z-scores for the cardiac dimensions were calculated, and each variable for fetuses and infants was tested using a two-sample t test between patients with and without coarctation. We excluded fetuses with TAPVR, Shone complex, interrupted aortic arch, Ebstein anomaly or HLHS. Of the 31 fetuses with in utero right heart enlargement, 11 had coarctation postnatally and 20 did not have coarctation. We compared the fetal and newborn cardiac dimensions between the groups. The mean fetal carotid-subclavian index (CS Index) was 0.7 mm with coarctation compared with 1.1 mm without coarctation (p < 0.0001). The mean difference in diameter z-scores for fetal aortic isthmus (p < 0.0001), mitral valve (<0.001) and aortic valve (p < 0.009) was also significantly different. Similar significant differences were noted postnatally in the diameters of the cardiac dimensions between the coarctation and no-coarctation group: CS index (p < 0.0001), aortic isthmus (p < 0.0002) and aortic valve annulus (p < 0.007). A spectrum of diagnoses was found postnatally in fetuses with right heart enlargement, including a normal heart. The likelihood of identifying fetuses with coarctation of the aorta and planning for postnatal management can be refined by noninvasive screening measurements. A smaller CS index and smaller diameters of the aortic isthmus, mitral valve and aortic valve were significantly associated prenatally (p < 0.05) with coarctation of the aorta versus without coarctation and might be useful in prenatally diagnosing coarctation of the aorta. Postnatally, these measurements are reproducible. This is the first study utilizing these specific measurements to diagnose coarctation prenatally.