Infection of the placenta can lead to pregnancy complications as well as fetal and maternal disease and death. We developed a novel system to study placental infections using murine fetal placental progenitor cells and the bacterial pathogen Listeria monocytogenes. In the mature placenta fetal progenitor cells fuse to form a large surface (syncytium) that is bathed in maternal blood and mediates nutrient and gas exchange between maternal and fetal circulation. We found that the syncytium resists physical deformation, and that its unusual cytoskeletal organization contributes to its elasticity. Weakening of its elastic properties correlated with increased susceptibility to infection. Our study presents a novel system to study placental infections, and provides new insights into the nature of the placental barrier.