In the three-vessel-trachea view, the sizes of the vessels should be equal.

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In the three-vessel-trachea view, the vessels you see are the main pulmonary artery, the aorta, and the superior vena cava, and their sizes reflect the different flows they carry in fetal circulation. The pulmonary artery usually appears the largest because it handles a large portion of right heart output and, via the ductus arteriosus, feeds systemic circulation. The aorta is typically smaller than the pulmonary artery, and the superior vena cava is the smallest of the three. Because they carry different volumes and have different anatomical roles, their calibers are not expected to be equal. Therefore the idea that their sizes should be equal is not correct. Minor variation can occur with gestational age and image technique, but a pattern of equal sizes is not a characteristic of a normal three-vessel-trachea view.