Preterm labor is a major cause of and infant mortality and long-term medical complications in offspring. Premature activation of the maternal immune system may stimulate cervical dilatation and activation of the uterine smooth muscle, leading to preterm delivery. Normal labor involves an increase in the expression of uterine-derived pro-inﬂammatory cytokines and chemokines that activate maternal peripheral leukocytes. Leukocytes then increase within the uterus and stimulate the cervical and uterine conditions that lead to delivery of the baby. Better characterization of these biochemical and cellular events may provide information that can be used to screen or predict the risk of preterm labor.