The normal umbilical cord coil index is one coil/5 cm, i.e., 0.2 +/- 0.1 coils completed per cm. We report the frequency and clinical correlations of abnormally coiled cords among 1329 cases referred to our placental pathology services. Twenty-one percent of cords were overcoiled and 13% were undercoiled. Abnormal cord coiling was seen at all gestational ages. Principal clinical correlations found in overcoiled cords were fetal demise (37%), fetal intolerance to labor (14%), intrauterine growth retardation (10%), and chorioamnionitis (10%). For undercoiled cords, the frequencies of these adverse outcomes were 29%, 21%, 15%, and 29%, respectively. Abnormal cord coiling was associated with thrombosis of chorionic plate vessels, umbilical venous thrombosis, and cord stenosis. Thus, abnormal cord coiling is a chronic state, established in early gestation, that may have chronic (growth retardation) and acute (fetal intolerance to labor and fetal demise) effects on fetal well-being. The cause of abnormal cord coiling is not known. Its effects on neurological status of survivors are also unknown. Antenatal detection of abnormal cord coil index by ultrasound could lead to elective delivery of fetuses at risk, thereby reducing the fetal death rate by about one-half. We recommend that the cord coil index become part of the routine placental pathology examination.