Sheep farming normally occurs in areas with limited vegetation. Climate change is placing more pressure on farming in these challenging environments. There is limited research on factors limiting lamb survival in marginal environments. The objective of this study was to characterize fetal and maternal factors associated with low (<80%) lamb survival in Patagonia. Corriedale ewes, synchronized, superovulated and mated to Suffolk rams and managed under commercial grazing conditions were used. Single- (n = 8) and twin-bearing (n = 8) ewes were euthanized at d 140 of gestation and mass of the fetal and placental components and ewe carcass traits were obtained. The effect of litter size (single vs twin) on maternal and fetal characteristics was analyzed using ANOVA. Maternal live weight was similar between twin- and single-bearing ewes (60.4 ± 1.6 vs 61.4 ± 1.6 kg, P = 0.7), while twin-bearing ewes had lower carcass weight compared with single-bearing ewes (18.9 ± 0.6 vs 20.7 ± 0.6 kg, P = 0.05), associated with a trend for reduced loin area (11.8 ± 0.7 vs 13.8 ± 0.7 kg, P = 0.07), but similar carcass fat depth (Fat GR: 4.5 ± 1.1 vs 4.12 ± 1.1 mm, P = 0.8; Fat C: 3.8 ± 0.6 vs 3.8 ± 0.6 mm, P = 1.00). Twin fetuses were lighter (3.24 ± 0.11 vs 4.18 ± 0.14 kg, P < 0.01), with proportionally lighter organ and leg muscle weight (P < 0.05), a tendency for less perirenal fat (9.15 ± 0.64 vs. 10.99 ± 0.78, P = 0.07) and disproportionately lighter adrenal glands (P = 0.05), M. adductor (P = 0.02) and M. gluteus (P = 0.03) compared with singletons. Total placentome weight (256.2 ± 10.6 vs 391.5 ± 14.9 g, P < 0.01) and number (49.6 ± 4.0 vs 75.6 ± 5.7, P < 0.01) were lower in twin- comparer to single-bearing ewes. Reduced placental mass and reduced fetal weight in twins compared with singletons is consistent with prior studies. The greater mobilization of skeletal muscle but not fat in twin-bearing ewes is likely a mechanism to enhance protein supply to the fetuses. Compared with studies in less marginal environments, the fetuses (both singles and twins) in this study had a higher percentage of perirenal fat (0.3% vs 0.2%), which may indicate an adaptive response, to enhance fetal survival.