Dietary composition during pregnancy influences fetal and adult phenotype but its effects on placental phenotype remain largely unknown. Using molecular, morphological and functional analyses, placental nutrient transfer capacity was examined in mice fed isocaloric diets containing 23%, 18% or 9% casein (C) during pregnancy. At day 16, placental transfer of glucose, but not methyl-aminoisobutyric acid (MeAIB), was greater in C18 and C9 than C23 mice, in association with increased placental expression of the glucose transporter Slc2a1/GLUT1, and the growth factor Igf2. At day 19, placental glucose transport remained high in C9 mice while MeAIB transfer was less in C18 than C23 mice, despite greater placental weights in C18 and C9 than C23 mice. Placental System A amino acid transporter expression correlated with protein intake at day 19. Relative growth of transport verses endocrine zones of the placenta was influenced by diet at both ages without changing the absolute volume of the transport surface. Fetal weight was unaffected by diet at day 16 but was reduced in C9 animals by day 19. Morphological and functional adaptations in placental phenotype, therefore, occur to optimise nutrient transfer when dietary composition is varied, even subtly. This has important implications for the intrauterine programming of life expectancy.