Fetomaternal immune tolerance is required to prevent fetal rejection during 
pregnancy; simultaneously, the maternal immune system must also protect the 
fetus from harmful endogenous and exogenous antigens, including those associated 
with viral and bacterial infections. Therefore, a delicate immune balance is 
critical for the maintenance of a successful pregnancy, while disruption of this 
balance can induce complications such as implantation failure, miscarriage, 
preterm birth/labor, preeclampsia and fetal growth restriction. While the 
adaptive immune system is critical for the development of tolerance, this review 
summarizes evidence that modulation of the innate immunity is also essential for 
achieving this delicate balance between tolerance and protection. Canonical 
cells of the innate immune system, including dendritic cells, macrophages, 
natural killer cells and invariant natural killer T cells, contribute to the 
modulation of the immune balance at the fetomaternal interface. The newly 
identified myeloid-derived suppressor cells and innate lymphoid cells also 
appear important for successful reproduction. Moreover, it is possible that 
sterile inflammation facilitates complications of pregnancy via the innate 
immune system. In this review, the characteristic features and functions of 
innate immune cells in fetomaternal immunity and their contributions to 
pregnancy complications related to sterile inflammation are discussed. These 
insights on innate immune system function in reproduction may provide new 
perspectives for understanding the mechanisms of fetomaternal tolerance and the 
etiology of pregnancy complications.