Natural killer (NK) cells present in large quantities, accounting for 70% of the maternal immune cells during the first trimester of pregnancy. With the development of single-cell sequencing technology, the researchers identified three groups of uterus NK cells with different phenotypes and functions in human uterus. decidual NK1 (dNK1) subsets produce growth promoting factors which regulate T helper type 2(Th2)cell immunity and thus facilitate pregnancy. dNK2 and dNK3 subsets secrete inflammatory cytokines that are regulators of Th2 cell immunity, not conductive to establishing and maintaining pregnancy. Single-cell sequencing studies in mice revealed a new subset of uNK (CYP26A1+uNK) cells that play an important role in embryo implantation and immune regulation.