From this radiograph, you can determine that this infant was born at term. The humeral head epiphysis (orange arrows) begins to ossify at 37 weeks of gestation, and about 40% of infants born at 40 weeks show ossified proximal humeral epiphyses. By 42 weeks gestation, 90% are ossified. Another clue you can use to determine the gestational age of an infant is to look at the molar enamel ossification (blue arrows). The first molar enamel begins to ossify its enamel at 33 weeks of gestation, and the second molar crown ossifies at 36 weeks of gestation. The sternum is well ossified at term and normally develops from 5 ossification segments (red arrows), including the manubrium and xyphoid. However, in many cases the xyphoid does not completely ossify, leaving 4 centers. We can see the centers clearly here because the baby was rotated on the film when it was taken. The ribs on the right look longer, because the baby is angled right side back. This throws the sternum off the midline to the right. Infants are more cylindrical in shape than adults. Their transverse and anterior-posterior diameters are about equal. The ribs are horizontally oriented. The heart looks relatively larger, and can be up to 65% of the width of the thorax. The thymus (black arrowheads) is normally very prominent. A large thymus is a sign of health, and the thymus decreases in size with stress.