We investigated the effects of using intrapartum electronic fetal monitoring in all pregnancies, as compared with using it only in cases in which the fetus is judged to be at high risk. Predominant risk factors included oxytocin stimulation of labor, dysfunctional labor, abnormal fetal heart rate, or meconium-stained amniotic fluid. This prospective alternate-month clinical trial took place over a 36-month period during which 34,995 women gave birth. In alternate months, either 7 (for "selective monitoring") or 19 (for "universal monitoring") fetal monitors were made available in the labor and delivery unit. During the "selective" months, 6420 of 17,409 women (37 percent) were electronically monitored, as compared with 13,956 of 17,586 women (79 percent) during the "universal months." Universal monitoring was associated with a small but significant increase in the incidence of delivery by cesarean section because of fetal distress, but perinatal outcomes as assessed by intrapartum stillbirths, low Apgar scores, a need for assisted ventilation of the newborn, admission to the intensive care nursery, or neonatal seizures were not significantly different. We conclude that not all pregnancies, and particularly not those considered at low risk of perinatal complications, need continuous electronic fetal monitoring during labor.