Preterm birth is a major cause of perinatal morbidity and mortality. Intrauterine infection/inflammation is associated with a majority of preterm labor and birth cases. Despite decades of studies recognizing a strong association between infection/inflammation and preterm birth, no effective method of preventing infection-induced premature labor and delivery is yet available. Importantly, the mechanisms by which intrauterine infection/inflammation may contribute to preterm birth are not known. Based on our observations with human gestational tissue to highlight the role of IL-10 in normal and comprimised pregnancy outcomes, we have performed experiments with syngeneic and allogeneic pregnant IL-10-/- mice or congenic wild type mice. Pregnancy outcomes were assessed in response to i.p. administration of low doses of lipopolysacchade (LPS) on gd 14. The mice were allowed to deliver or were sacrificed on gd 16 for isolation of uterine immune cells for functional studies or collected tissue for histological analysis. Attempts were made to prevent preterm parturition. LPS-treated IL-10-/-, but not wild type mice, displayed a significant acceleration in time of delivery, on gd 16.5 compared to gd 19.6 for wild type controls. The premature delivery observed in LPS-treated IL-10-/- mice was associated with an increase in the number of uterine NK (uNK) cells. These cells also displayed a dramatic infiltration of the placenta with a perivascular localization. uNK cells appear to be responsible for the induction of preterm birth in these mice as depletion of NK cells completely restored normal length of gestation. Moreover, neutralization TNF-α also rescued the premature delivery. Taken together, our results for the first time demonstrate that IL-10 deficiency and uterine NK cell cytotoxic activation link intrauterine inflammation to preterm parturition.