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The present studies assessed the effect of hypobaric hypoxia on fetal lamb growth in high-altitude (HA) and low-altitude (LA) native ewes. Growth patterns of fetal biparietal diameter (BPD), abdominal diameter (AD) and thorax height (TH) were described by consecutive ultrasound measurements throughout the entire pregnancy. Three groups of animals were used: (1) pregnant LA ewes kept at LA (control; 'LL' group); (2) pregnant LA ewes moved to HA immediately after confirmation of pregnancy ('LH' group); and (3) pregnant HA ewes kept at HA throughout the entire pregnancy ('HH' group). The slope of the BPD curve was higher in LL fetuses followed by that in LH fetuses. During the last month of pregnancy, TH was higher in LH and HH fetuses, whereas AD was higher in LL than in LH fetuses. The length of gestation was longer in HH ewes (153.2 +/- 4.3 days) than in LH and LL ewes (146.0 +/- 5.5 and 145.0 +/- 3.0 days, respectively). Bodyweight at birth was higher for LL newborns (4.2 +/- 0.3 kg) than for LH and HH newborns (3.0 +/- 0.5 and 3.2 +/- 0.8 kg, respectively), whereas placental weight was higher in the HH group (396 +/- 80 g) than in the LH (303 +/- 64 g) and LL (280 +/- 40 g) groups. In conclusion, an HA environment modifies fetal growth and pregnancy outcome with the magnitude of effects depending on the time of residence at HA.