What are the roles of macrophages and monocytes in human pregnancy?

During pregnancy, the maternal immune system is challenged by the semi-allogeneic fetus, which leads to systemic and local immunity. Systemic immunity, including enhanced innate immunity with increased activation of monocytes, is induced by various placental factors. Maternal immune adaptations are most evident at the feto-maternal interface, where macrophages are enriched and communicate with various decidual leukocytes. These cells are not only contributing to the protection of the growing fetus from microorganisms, but also aiding placental development by promoting trophoblast invasion and spiral artery remodeling, and the parturition process. Thus, monocytes and macrophages concurrently play important roles throughout the trimesters. Dysregulation of these cells may thus lead to pregnancy complications, such as pre-eclampsia and preterm labor. In this review, monocytes and macrophage subsets and their roles in normal and pathological pregnancies are reviewed.