Objectives: It has been hypothesized that male fetuses down regulate placental growth during periods of accelerated fetal growth. We aimed to investigate this, and determine whether sexual dimorphism was apparent in the spiny mouse placenta. We hypothesized that expression of fetal growth promoters would be higher in placentas of males, whereas genes involved in placental structural development would be more highly expressed in placentas of females. Methods: Spiny mouse dams, a precocial rodent with an in utero endocrine milieu dissimilar from other rodents, but akin to humans, were sacrificed at gestational ages 15-37 (term = 39 days). Placentas were collected and processed for histology or qPCR analysis of selected genes (GCM1, MAP2K1, SLC2A1, NR3C1, IGF1, IGF1R). Results: Fetal and placental weights were similar for both sexes. Placentas of female fetuses had less spongy zone (PSEX <0.0001), and more labyrinth (PSEX <0.0001) than males. Early placenta and labyrinth expression of SLC2A1 was higher in males than females (PSEX <0.05). Labyrinthine IGF1R remained constant until term in the female, compared with male where expression increased until term. Peak MAP2K1 expression occurred earlier in the male placenta than the female. Spongy zone SLC2A1 remained constant until term in the female, compared with male where expression increased until term. Conclusions: The spiny mouse is a species that exhibits sexually dimorphic placental development. We suggest that these sex differences in placental gene expression and structure may underlie or compound the male vulnerability to a sub-optimal in utero environment.