High altitude residence causes fetal growth restriction (FGR) during pregnancy in lowland mammals. Highland-adapted mammals do not experience this altitude-dependent FGR, suggesting that evolutionary adaptation has provided some physiological protection. However, the specific mechanisms by which highland-adapted mammals preserve fetal growth at altitude remain unknown. We hypothesized that highland-adapted populations protect fetal growth through structural changes to the placenta that increase surface area for maternal-fetal nutrient and gas exchange. We tested this hypothesis using deer mice (Peromyscus maniculatus), from populations native to low [400 m, Lincoln, NE] and high [4300 m, Mt. Evans, CO] altitudes. We predicted structural adaptation would occur via increases to the relative size of the labyrinth zone (LZ), the layer within the rodent placenta where nutrient and gas exchange occur. Placentas were collected from lowland and highland deer mice undergoing pregnancy under normobaria or hypobaria (60 kPa) to understand how hypoxia-dependent structural plasticity might interact with adaptive remodeling of the placenta (N = 5-7 per strain and treatment). Using immunohistochemistry, we quantified the size of each placental zone. Our preliminary results showed that highlanders have relatively larger placental arteries and LZs under both normobaria and hypobaria (P < 0.05 in generalized linear mixed models), suggesting that blood delivery and area for exchange (as determined by the LZ size) may protect fetal growth in highlanders. Future work will pair histological characterization of placental structure with transcriptomics to guide a mechanistic understanding of how placentation constrains fetal growth under hypoxia.