The normal newborn chest has a trapezoidal shape in AP films. On normal inspiration the hemidiaphragms in an AP chest film are at the level of the sixth rib anteriorly and the eighth or ninth rib posteriorly. The lungs are more radiolucent than those of older children. Air bronchograms may be seen in the medial thirds of the lungs [1].
        The heart is more spherical in shape than in an older child. The cardiothoracic ratio of newborns has a wide range. The upper limit is about 0.65 [1], beyond which cardiomegaly should be considered. The aorta and pulmonary arteries are difficult to identify because of the thymic shadow.
        The thymus is usually seen in newborns. It has right and left lobes and is in the anterosuperior mediastinum. The border of the heart and the inferior border of the thymus may blend (Figure 1), or there may be a notch at the junction of the border of the heart with the inferior border of the thymus  (Figures 2 and 3). Sometimes the thymus has an inferior margin with an acute angle which produces a sail sign. If there is rotation in films of patients with a sail sign, the thymus may appear shifted laterally, which may be incorrectly interpreted as upper lobe pneumonia (Figure 4) [2]. Occasionally gentle undulation is seen of the entire lateral edge of the gland, a result of adjacent rib compression. This is termed the wavy thymic sign [2] (Figure 3).
 
 

REFERENCES
1. Hedlund GL, Griscom NT, Cleveland RH, Kirks DR. Respiratory system. In: Kirks DR, Griscom NT, editors. Practical pediatric imaging: diagnostic radiology of infants and children. 3rd ed. Philadelphia: Lippincott-Raven, 1998:690-3.
2. Swischuk LE. Respiratory system. In: Imaging of the newborn, infant, and young child. 4th ed. Baltimore: Williams & Wilkins, 1997:1-25.