Mabuya skinks have the most specialized allantoplacenta and the greatest degree of placentotrophy known among the Reptilia. Their recently ovulated eggs are microlecithal (1-2 mm) and lack fatty yolk platelets; thus, virtually all of the nutrients for embryonic development must pass across the placenta. We quantified the net uptake of nutrients during gestation in an Andean population of Mabuya and compared these results with other matrotrophic skinks and eutherian mammals. Total dry and wet masses, ash and organic content, ions (calcium, potassium, sodium, magnesium, iron), lipids (cholesterol, vitamin E and fatty acids in the major lipid classes-triacylglycerol, phospholipids, cholesteryl ester, free fatty acids), nitrogen (an index of protein) were measured during the different developmental stages throughout gestation, and in neonates. A significant net uptake of inorganic and organic matter was found. This uptake begins slowly during early gestation but accelerates during the last third of gestation when the growth of the embryo is maximal and the allantoplacenta has developed its greatest complexity. The drastic reduction of egg size in this clade is related to the great reduction in the contribution of lecithotrophic nutrients to the embryo, an obligatory placentotrophy from early developmental stages, and the highest placental complexity known in the Reptilia. All of these features converge with features found in eutherian mammals.