Skinks of Mabuya genus exhibit the most specialized allantoplacenta among squamates (type IV), and the greatest degree of placentotrophy known in Reptilia. They ovulate microlecithal eggs (1–2 mm) that lack fatty yolk platelets; thus, it is suggested that virtually all of the nutrients for embryonic development are obtained by placental means. To test this inference, the net uptake of nutrients during gestation in an Andean population of Mabuya was quantified and compared with other oviparous and viviparous lizards, matrotrophic skinks, and eutherian mammals. Ionic, protein and lipid contents of recently ovulated eggs and neonates were measured. A significant net uptake of water, ions (calcium, potassium, sodium, magnesium), lipids, nitrogen (an index of protein), and dry matter was observed during development. Thus, in Mabuya the drastic reduction of egg size is related to the great reduction in the contribution of lecitotrophic nutrients to the embryo, an obligatory placentotrophy from early developmental stages, and the highest placental complexity known in Reptilia. All of these features converge with similar features found in eutherian mammals. Both clades evolved similar reproductive patterns and morphological features in their complex chorioallantoic placentae, which supplies all the nutrients for embryonic and fetal development.