Tibetan protection from intrauterine growth restriction (IUGR) and reproductive loss at high altitude

Chronic hypoxia at high altitude restricts fetal growth, reducing birth wei ght and increasing infant mortality. We asked whether Tibetans, a long-resi dent high-altitude population, exhibit less altitude-associated intrauterin e growth restriction (IUGR) and prenatal or postnatal reproductive loss tha n Han (ethnic Chinese), a group that has lived there for a shorter period o f time. A population sample was obtained, comprising 485 deliveries to Tibe tan or Han women over an 18-month period at 8 general hospitals or clinics located at 2,700-4,700 m in the Tibet Autonomous Region, China. Birth weigh t, gestational age, and other information were recorded for each delivery. Prenatal and postnatal mortality were calculated using information obtained from all pregnancies or babies born to study participants. Tibetan babies weighed more than the Han, averaging 310 g heavier at altitudes 2,700-3,000 m (95% CI = 126, 494 g; P < 0.01) and 530 g heavier at 3,000-3,800 m (210, 750 g; P < 0.01). More Han than Tibetan babies were born prematurely. Pren atal and postnatal mortality rose with increasing elevation and were 3-fold higher across all altitudes in the Han than the Tibetans (P < 0.05). Tibet ans experience less altitude-associated IUGR than Han and have lower levels of prenatal and postnatal mortality. When the relationships between birth weight and altitude are compared among these and other high-altitude popula tions, those living at high altitude the longest have the least altitude-as sociated IUGR. This may suggest the occurrence of an evolutionary adaptatio n. (C) 2001 Wiley-Liss, Inc.