Background: Studies performed in Andean populations living in high altitude, indicate that the reduced availability of oxygen could be associated to both a fetal growth retardation and a lower birth weight. These variables are predictive of morbidity and mortality during the first year of life. Aim: To study perinatal variables of newborns of mothers living at contrasting altitudinal levels, harboring different degrees of Aymara ancestry. Subjects and Methods: Review of medical records of 5295 women whose deliveries occurred between February 2004 and August 2010. Information was obtained on place of residence, grouped into two categories: coast (150 to 3,000 m) and high plateau (3.000 to 4.300 m), ancestry was estimated using number of Aymara surnames that were homologated to percentages of Amerindian admixture, gestational age, birth weight, height, head circumference and obstetric variables. Results: Gestational age showed a tendency to increase and birth weight, height and head circumference to decrease with altitude of residence. Only weight reached statistical significance. Women with Aymara ancestry gave birth to children with a significantly higher gestational age, weight and cranial circumference. Conclusions: Altitude of residence is related to a decrease in perinatal variables that proved to be less pronounced in newborns of mothers with a higher degree of Aymara ancestry. Results suggest a genetic adaptation to hypoxia that could be related to candidate genes linked to the capture, transport or utilization of oxygen.Background: Studies performed in Andean populations living in high altitude, indicate that the reduced availability of oxygen could be associated to both a fetal growth retardation and a lower birth weight. These variables are predictive of morbidity and mortality during the first year of life. Aim: To study perinatal variables of newborns of mothers living at contrasting altitudinal levels, harboring different degrees of Aymara ancestry. Subjects and Methods: Review of medical records of 5295 women whose deliveries occurred between February 2004 and August 2010. Information was obtained on place of residence, grouped into two categories: coast (150 to 3,000 m) and high plateau (3.000 to 4.300 m), ancestry was estimated using number of Aymara surnames that were homologated to percentages of Amerindian admixture, gestational age, birth weight, height, head circumference and obstetric variables. Results: Gestational age showed a tendency to increase and birth weight, height and head circumference to decrease with altitude of residence. Only weight reached statistical significance. Women with Aymara ancestry gave birth to children with a significantly higher gestational age, weight and cranial circumference. Conclusions: Altitude of residence is related to a decrease in perinatal variables that proved to be less pronounced in newborns of mothers with a higher degree of Aymara ancestry. Results suggest a genetic adaptation to hypoxia that could be related to candidate genes linked to the capture, transport or utilization of oxygen.