POX Range and the Threshold for Screening Major CHD in Neonates at Different Altitudes

Inclusion Criteria: For healthy newborns: * Healthy, asymptomatic, singleton term infants (37-41 weeks gestation); * Appropriate for gestational age (using the international WHO's intrauterine growth curve to evaluate); * Only newborns with Apgar score ≥ 7 in 1 minute and 5 minutes were selected; * There is no known congenital heart disease based on the ECHO result , no murmur, no obvious signs (no fever, no tachycardia or respiratory symptoms at the time of admission). For major CHD newborns: * Echocardiography clearly diagnosed the following congenital heart diseases: Aortic stenosis, Double outlet of right ventricle, Ebstein's malformation, Left cardiac hypoplasia syndrome, Aortic arch disconnection, Pulmonary atresia, Single ventricle, Tetralogy of Fallot, Complete anomalous pulmonary venous drainage, Transposition of great arteries, Tricuspid valve atresia, and Single trunk of arteries and other major CHD based on Ewer's denition. Exclusion Criteria: For healthy newborns: * Newborn with caesarean section; newborn with smoking mother; newborn with definite congenital heart disease before discharge. For major CHD newborns: * There is no exclusion criteria for major CHD.