Newborn infants, especially those in the neonatal intensive care unit, tend to go through a transition period in early life including complex perfusion changes. Haemodynamic monitoring allows for early detection of diseases, thus improving outcomes and can be life-saving. A reliable modality of hemodynamic monitoring requires early, direct, non-invasive and continuous detection of irreversible damage caused by insufficient tissue perfusion and oxygenation. PPI has been embedded in the latest generation of pulse oximeters, which obtain data via a sensor attached to infants' palm or sole and emit 940 nm near-infrared light. Different tissues and blood vessels absorb different amounts of light, with the amount of light absorbed by pulsatile tissues-dynamic changes of arterial blood flow, and non-pulsatile tissues-venous blood, muscle and other tissues, being detected. The amount of light absorbed by non-pulsatile tissue is constant. PPI is the percentage of the former to the latter.1 It reflects the real-time change of peripheral blood flow and is an evaluation of pulse intensity at specific monitoring sites.2 In the newborn population, GA has a great impact on physiology. The early postnatal PPI values of newborns at different GAs may also be different.1 Therefore, the range and thresholds of PPI values for detecting diseases in newborns should be modified according to different GA.1 In addition, studies which investigated PPI were previously conducted at sea level but lacked data from other altitudes.3 The purpose of the brief report is to demonstrate the effect of GA on PPI in the early postnatal period at different altitudes. A multi-centre and cross-sectional study was conducted at six institutions with five altitudes in China. The altitudes distribution of the institutions are as follows: Low altitude (Hainan Women and Children's Medical Center, 4 m); Mild altitude (People's Hospital of Xinjiang Uygur Autonomous Region, 800 m); Moderate altitude (Luchun County People's Hospital, 1640 m, and Yan'An hospital affiliated to Kunming Medical University, 1891 m); High altitude (Tibet autonomous region people's hospital, 3658 m) and Higher altitude (Golog Tibetan Autonomous Prefecture People's Hospital, 4200 m). Consecutively newborn infants were enrolled from 1 February 2020 to 15 April 2021. The mothers of these newborn infants lived locally throughout their pregnancy. All newborn infants underwent standard postnatal care. Newborn infants requiring respiratory support, having congenital malformations, or those who did not have complete data were excluded. The Research Ethics Committee of Children's Hospital of Fudan University in Shanghai, China ([2020]-132) approved the study, and the protocol was registered on Clinical Trials.gov (Registry No. NCT04238104). Our original study was to observe saturation of pulse oxygen values in healthy full-term newborn infants. During the study period, we also measured PPI values. Signed informed consent was obtained. A Masimo Radical-7 (masimo, USA) motion-resistant pulse oximeter was used to measure PPI values at 12 h of life by a well-trained investigator. This time point was selected to obtain the blood perfusion status of the newborn at the relatively early postnatal age. Five consecutive PPI values were recorded at 1-min intervals with the average value derived. The measurements were obtained from normothermic infants in the supine position and while settled in either the awake or asleep state on the palm of the right hand. Other covariates, such as ethnicity, Apgar score, mode of delivery, appropriateness for gestational age, sex and maternal history were also collected. Maternal history data included maternal hypertension and maternal diabetes. The training before data collection and the supervision during the collection process have carried out strict quality control.1 PPI data were continuous measures which were not normally distributed and were consequently log-transformed before regression analysis was carried out. The multi-variable linear model was used to investigate the effect of GA on early postnatal PPI at different altitudes, adjusting potential confounding factors. All statistical analyses of data were performed by SPSS 26 software (IBM Corp). Two-tailed test with a significant level p < 0.05 was used in our study. A total of 4971 newborn infants were delivered during the study period, of which 3665 were eligible and included in the final analysis. The median GA was 39+0 days/weeks and the median PPI was 1.80 (interquartile range, IQR 1.20, 2.60). Taking confounding factors into account, with the increase of each week GA, the value of PPI increased 1.040 (95% CI 1.018, 1.061) times at low altitude (Table 1). In low-altitude areas, the lower the GA, the less the cardiac output, and the lower the PPI. Another cohort study4 that used two non-invasive cardiovascular assessments found differences in cardiovascular adaptive responses in preterm and term newborn infants. This suggested that GA may affect PPI, which was similar to our findings at low altitude. At other altitudes, GA did not affect PPI at all. The reason may be that altitude itself has a greater impact on infants, while in contrast, the impact of GA on PPI is trivial. Our study is also the first to suggest that maternal hypertension may have an impact on infant PPI (Table S1).5 This needs to be confirmed by studies in the future. Maternal hypertension may lead to decreased placental perfusion, decreased function, intrauterine hypoxia and insufficient perfusion in the postnatal period. Some studies compared the PPI, and found that values in term and preterm groups were different (p < 0.05). This comparison should consider other confounding factors and adjust to obtain the specific changes in PPI according to GA. We further studied, considered confounding factors and used a multivariable linear model to estimate the impact and degree of GA on PPI. Additionally, there were no previous studies on PPI in higher altitude areas before. Our findings validated the hypothesis that the effects of GA on PPI at low altitude may not be applicable at other altitudes. Our research included a large population in a multi-centre study, but only included PPI measurements at 12 h. In the future, we need to explore different PPI cut-off values for identifying sick infants at different GAs. We thank all the newborn babies and their parents who participated in this study. We thank all professionals involved in the Neonatal Congenital Heart Disease Research Group in Plateau Areas of China. We also thank Mr Conway Niu from King Edward Memorial Hospital, Western Australia for his review of the manuscript. None. Supported by the National Key Research and Development Project of China (2021YFC270100, 2016YFC1000500), Shanghai Science and Technology Commission (21002411900), and the Chinese Academy of Medical Sciences Innovation Fund for Medical Sciences (2019-I2M-5-002). Table S1 Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.