Macrophages perform many specific functions including host defense, homeostasis, angiogenesis, and tissue development. Macrophages are the second most abundant leukocyte population in the non-pregnant endometrium and pregnant decidua and likely play a central role in the establishment and maintenance of normal pregnancy. Importantly, aberrantly activated uterine macrophages can affect trophoblast function and placental development, which may result in various adverse pregnancy outcomes ranging from pre-eclampsia to fetal growth restriction or demise. Only by fully understanding the roles of macrophage in pregnancy will we be able to develop interventions for the treatment of these various pregnancy complications. This review discusses the general origin and classification of monocytes and macrophages and focuses on the phenotype and functional roles of decidual macrophage at the maternal–fetal interface in normal pregnancy, as well as discussing the potential contribution of the abnormal state of these cells to various aspects of pregnancy pathologies.