Protection from intrauterine growth retardation in Tibetans at high altitude.

Intrauterine growth retardation has long been recognized at high altitude. Since growth-retarded babies have a decreased chance of survival, intrauterine growth retardation would be expected to have been selected against in populations long resident at high altitude. We have previously reported that Tibetan babies born at 3,658 m weighed more than their North or South American altitude counterparts. This study sought to determine whether Tibetans were protected from altitude-associated intrauterine growth retardation. We compared birth weights in Tibetans living at low altitude in Kathmandu, Nepal (elevation 1,200 m), or at high altitude in Lhasa, Tibet Autonomous Region, China (elevation 3,658 m). Birth weights were similar in 45 low-altitude and 34 high-altitude Tibetan births regardless of whether all infants or only full-term births were considered, or whether birth weight was adjusted for variation in maternal parity, gestational age, and infant sex. In comparison with literature observations, the altitude-associated difference in birth weight was smallest in Tibetans, intermediate in South America, and greatest in North America. These data support the hypothesis that Tibetans are protected from altitude-associated intrauterine growth retardation and suggest that selection for optimization of birth weight at high altitude has occurred in Tibetans.