During pregnancy, littermates compete to extract maternal resources from the placenta. Unequal extraction of resources leads to developmental differences among offspring and thus within-litter variation in offspring mass. Because competition among littermates can be stronger among half-sibs, multiple paternity may represent an adaptive strategy allowing females to increase within-litter phenotypic variation among offspring when facing variable environments. Wild boar (Sus scrofa) females produce large litters with diversified offspring in terms of body mass. Additionally, multiple paternities within a litter have been observed in this promiscuous species. One can hypothesize that multiple paternity represents the mechanism by which females increase within-litter phenotypic variation. Combining long-term monitoring data with paternity analyses in a wild boar population, we tested whether the increase in the number of fathers within a litter explained the increase in within-litter variation in offspring mass observed in large litters. We showed that heavy females mated earlier during the rut, produced larger litters with a higher number of fathers and more variable fetus mass than lighter females. Within-litter variation of offspring mass increased with gestation stage and litter size, suggesting differential allocation of maternal resource among offspring “in utero.” However, we found only a weak paternal effect on offspring mass and no direct effect of the number of fathers on the within-litter variation in offspring mass. These results indicate that differential maternal allocation to offspring during pregnancy is unlikely related to paternal identity in this species.