Porcine reproductive and respiratory syndrome virus (PRRSV) crosses the placenta most easily in the last third of gestation. Further, PRRSV does not replicate in preimplantation embryos but does replicate in postimplantation embryos and fetuses. In the present study, it was aimed to find an explanation for these observations by localization and quantification of the macrophages carrying two entry mediators that play a crucial role in PRRSV replication, sialoadhesin (Sn) and CD163, in the implantation sites and organs of embryos/fetuses during gestation. Uterus and embryos or organs (liver, spleen, lungs) from fetuses were obtained from pregnant PRRSV negative sows at different days of gestation (20-35, 50-60, 70-80, 114) and the Sn(+) and CD163(+) macrophages were quantified. In endometrium and placentas, two macrophage subsets were observed: Sn(-)CD163(+) and Sn(+)CD163(+). The highest number of Sn(+) and CD163(+) macrophages was counted at 114 days of gestation. In the mid-gestation fetal placentas (50-60 days of gestation), most CD163(+) macrophages were Sn negative. The number of Sn(+) and CD163(+) macrophages in organs increased during gestation. In the liver, the Sn(+) and CD163(+) macrophages were most abundant (Sn(+): 8.1-48.7%; CD163(+): 22.0-55.0%); the lowest number of Sn(+) and CD163(+) macrophages was observed in the lungs (Sn(+): 0-15.2%; CD163(+): 4.0-19.3%). Double immunofluorescence staining revealed three macrophage subsets in the spleen: Sn(+)CD163(-), Sn(-)CD163(+) and Sn(+)CD163(+); and two macrophage subsets in the lungs: Sn(-)CD163(+) and Sn(+)CD163(+). In the liver, due to physiological presence of biotin, the double immune-fluorescence staining could not be performed. The present results show clear changes in the quantity of Sn(+) and CD163(+) macrophages in the placentas and organs of embryos/fetuses during gestation which most probably have a physiological basis. The absence of Sn on macrophages in the fetal placenta at mid-gestation might explain the difficulty for PRRSV to spread transplacentally at this stage of gestation.