Objectives: To describe fetal and placental development in the spiny mouse, and examine expression of genes that have key roles in the physical development and transport of nutrients in placentas of male and female fetuses.

Study design: Dams were killed at gestational ages from day 20 to term (39 days) and fetal and placental tissues were weighed. Placental tissue was processed for histology, or divided into spongiform and labyrinth zones and processed for expression of selected, non-imprinted, genes (GCM1, MAP2K1, SLC2A1, NR3C1, IGF1, and IGF1R) by real-time quantitative PCR.

Results: Fetal and total placental weights were similar for males and females at all time points examined, but the relative size of the placental labyrinth and spongiform zones were different between the sexes. Placentas of female fetuses possessed less spongiform zone and trophoblast and more labyrinth than male littermates, so that the labyrinth:spongiform and maternal blood sinusoid:trophoblast ratios were higher in females compared to males across gestation. Sexually dimorphic gene expression was observed for IGF1R, SLC2A1, and MAP2K1 in the placenta.

Conclusions: While the growth of male and female fetuses is similar, their placentas develop differently, suggesting that extraction of nutrients from maternal supplies and the placental contribution to male and female fetal growth may differ from early in gestation. The utilisation of different pathways to achieve fetal growth may, in part, underpin or compound the sexually dimorphic responses of the placenta to adverse in utero environments, which in turn may explain the increased morbidity and mortality observed for male offspring.