Clinical chorioamnionitis is a primary infection that involves the chorioamniotic membranes and the intraamniotic cavity. Microscopically, clinical chorioamnionitis is defined by bacteria and leukocytes that are found between the chorion and the amnion, and by isolation of bacteria from the amniotic fluid. Other terms used in the literature are intraamniotic infection, amnionitis, amniotic fluid infection, intrauterine infection, and intrapartum infection. However, clinical chorioamnionitis is distinct from histological chorioamnionitis, which is defined by the infiltration of polymorphonuclear cells into the placenta and choriamniotic membranes.

The clinical manifestations of chorioamnionitis include fever, maternal and fetal tachycardia, uterine tenderness, foul odor of the amniotic fluid, and leukocytosis. Although a combination of these signs may be present in most women with chorioamnionitis, they are by no means universal. Recent studies are usually based on the clinical criteria defined by Gibbs and Duff which include maternal fever of 37.8°C or greater as a mandatory sign, plus two or more minor signs (maternal tachycardia >100 beats/min, fetal tachycardia >160 beats/min, uterine tenderness, foul odor of the amniotic fluid and leukocytosis >15000/mm3).

Fever accompanies virtually all cases of clinical chorioamnionitis. The criteria that define maternal and fetal tachycardia exceed the 90th percentile and are discovered in a wide range (20–80%) of patients with intraamniotic infection. Leukocytosis can be a normal finding in women during labor without obvious signs of infection. The mean intrapartum white blood cell count is 12500±3900/mm3 and increases in linear fashion with an extending duration of labor.