Mothers and fetuses are expected to be in some degree of conflict over the allocation of maternal resources to fetal growth in the intrauterine environment. Variation in placental structure and function may be one way a fetus can communicate need and quality to its mother, potentially manipulating maternal investment in its favor. Whereas common marmosets typically produce twin litters, they regularly give birth to triplet litters in captivity. The addition of another fetus is a potential drain on maternal resource availability and thus a source of elevated conflict over resource allocation. Marmoset littermates share a single placental mass, so that differences in the ratio of fetal to placental weight across litter categories suggest the presence of differential intrauterine strategies of resource allocation. The fetal/placental weight ratio was calculated for 26 marmoset pregnancies, representing both twin and triplet litters, to test the hypothesis that triplet fetuses respond to intrauterine conflict by soliciting placental overgrowth as a means of accessing maternal resources. In fact, relative to fetal mass, the triplet marmoset placenta is significantly undergrown, with individual triplets associated with less placental mass than their twin counterparts, suggesting that the triplet placenta is relatively more efficient in its support of fetal growth. There still may be an important role for maternal-fetal conflict in the programming of placental structure and function. Placental adaptations that solicit potential increases of maternal investment may occur at the microscopic or metabolic level, and thus may not be reflected in the size of the placenta as a whole. Am J Phys Anthropol, 2008. © 2008 Wiley-Liss, Inc.