Fetal demand, shaped by factors such as number of fetuses, may alter placental regulation of exchange, even when maternal nutrition restriction is not overt. The marmoset is an interesting model in which to examine this aspect of placental function due to unique placentation that leads to multiple fetuses sharing a unified placental mass. We demonstrated previously that the triplet marmoset placenta exhibits significantly higher efficiency than does the twin placenta. Here, we test the hypothesis that this increased efficiency is due to increases in changes in the microscopic morphology of the placenta. Stereology was employed to analyze the microscopic architecture of placentas from twin and triplet pregnancies. Compartments of interest were the trabeculae, intertrabecular space, fetal capillaries, and the surface area of the maternal-fetal interface. Placentas from the two litters did not differ significantly in overall volume or individual volumetric compartments, but triplet placentas exhibited significant expansion of the trabecular surface area in comparison to twins (p=0.039). Further, the two groups differed in the isomorphy coefficient, with triplet placentas having a significantly higher coefficient (p=0.001) and potentially a more complex microscopic topography. Differences in the maternal-fetal interface may be due to developmental constraints on gross placental growth that occur earlier in gestation, such that the only option for maintaining sufficient access to maternal resources or signaling pathways late in gestation is via an expansion of the interface. Despite the significant increase in overall surface area, individual triplet fetuses are associated with much less surface area than are individual twins, suggestive of alterations in metabolic efficiency, perhaps via differential amino acid transport regulation.