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Birth weight and crown-heel length has been reported by many researchers to be reduced at high altitude. However, many of these studies lacked adequate control of maternal nutritional status which may be confounding the altitude differences. In addition, few studies have examined the sources of variation in birth weight at high altitude that may be related to differential maternal adaptation to the stresses of this hypoxic environment. In this report we test the hypotheses that altitude differences in fetal growth exist independent of maternal nutritional status, and that indigenous Amerindian (Quechua and Aymara) women deliver larger infants at high altitude than non-Indian women who were born and raised and completed a full-term pregnancy in the same altitude environment. Samples of 105 healthy mothers and infants from La Paz, Bolivia (3600m) and 77 from Santa Cruz, Bolivia (400m) are analyzed for altitude and ethnic variation after covariance in maternal stature, mid-arm soft tissue composition, parity and gestational age are controlled. Analysis of variance and covariance indicate significantly smaller infants born at high altitude and born to non-Indian women; also male infants are more affected by high altitude than female infants. It is concluded that ethnic group difference in pregnancy outcome reflects a better state of adaptation to high altitude in this healthy indigenous population, and that long-term genetic selection may be the most plausable explanation for these ethnic differences.