Maternal uterine natural killer cells nurture fetal growth: in medio stat virtus

•Maternal immune responses need not be generally suppressed during pregnancy.•Adequate activation of uterine natural killer cells impacts on birth weight.

Much research in reproductive immunology is preoccupied with maternal tolerance of the semi-allogeneic fetus. This inevitably leads to the assumption that the maternal immune system should be suppressed, similarly to the immunosuppression needed to avoid rejection of an allograft. However, the parallels with transplantation immunology are misleading, and we discuss how interactions between variable immune system genes expressed on maternal natural killer (NK) cells and on the fetal trophoblast modulate fetal growth. Exaggerated suppression or activation of maternal NK cells associates with both extremes of birth weight.