Non‐technical summary  Studies on mice using severe diets show alterations in placental function, and fetal and adult health. However, little is known about the effects of mild dietary variations on the placenta. We investigated placental growth and function in mice fed diets with similar energy, but small differences in protein and sugar content. We show that placental adaptations occur to help support fetal growth: reduced protein leads to increased glucose transport and transporter gene expression in late pregnancy; just prior to term, amino acid transport expression correlated with protein intake; the placental endocrine compartment was smaller with the least dietary protein and somewhat larger with slight reduction in protein. Placentas in mice fed the least protein were better adapted than those exposed to slight protein reduction. These results may provide a good index of conditions in the womb and have important implications for the pre‐birth programming of life expectancy.