To assess uterine activity before labor in patients delivering preterm, at term, and postterm, the maximum spontaneous contraction frequency per 10-minute window during the initial portion of antepartum fetal heart rate monitoring was analyzed. Patients with multiple gestation, third trimester bleeding, polyhydramnios, or premature rupture of membranes and those already diagnosed with preterm labor were eliminated from the study. Of the 2446 remaining patients (7247 antepartum fetal heart rate tests) who went into spontaneous labor, 237 did so before 37 completed weeks of gestation, 1077 entered labor at term (38 to 42 completed weeks), and 1132 did so after 42 weeks. There was a significant increase in maximum uterine activity per 10-minute window from 30 to 44 weeks of gestation (average 4.7% per week; r = 0.97, p less than 0.0001). When compared with patients delivering spontaneously at term, average maximum uterine activity per 10-minute window was greatest in the preterm labor group (p less than 0.05) and least in the postterm labor group (p less than 0.05). These differences were present for several weeks preceding the onset of spontaneous labor. All three groups showed a surge of uterine activity during the 3 days before the onset of spontaneous labor.