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kkh.com.sg The aim of this study is to establish the norms for pulmonary arterial diameters in the premature infants. One hundred and thirty cross-sectional echocardiograms were performed on 62 premature neonates (23.4 weeks to 36 weeks gestation) in the Neonatal Intensive Care Unit. Except for small atrial septal defects / patent foremen ovale (<3 mm) or patent ductus arteriosus (PDA), babies with structural heart defects were excluded. The weight at echocardiography ranges from 470 grams to 2445 grams, with a mean of 1157 grams. The diameter of the pulmonary annulus (PA), left pulmonary artery (LPA) and right pulmonary artery (RPA) were measured at peak systole at predetermined sites. Sizes of the atrial septal defect and PDA were also measured, if present. There was no difference in the diameter between the left and right pulmonary arteries (p=0.254, paired t-test) in the same patient. After controlling for weight, the mean diameters of the LPA and RPA were larger in patients with PDA (p=0.002) compared to those without PDA (p=0.002), while their pulmonary annulus were comparable in size (p=0.691). Between the gestational ages of 23 and 36 weeks, the diameter of PA, LPA and RPA correlated linearly with weight (Pearson R = 0.84, 0.82, 0.65 and 0.71, respectively; p<0.0005). Prediction graphs and regression equations are given. These normal ranges can be used for assessment of pulmonary artery diameters in premature neonates