The authors have developed a system to monitor meconium continuously during labour, using a flexible intrauterine probe which detects back-scattered light. The system provides a measurement of the meconium concentration of amniotic fluid during labour every 2 min, with a 60% prediction interval of +or-10 g l-1, and a 99% prediction interval of +or-30 g l-1 (clinically 'thick meconium' contains around 100 g l-1). The noise of the measurement is of the order of +or-10 g l-1, and the response to changes in the meconium concentration is from 40 to 120 s, depending on its configuration. The system also provides other obstetric variables such as fetal heart rate and uterine activity measurement, obtained from a fetal monitor. Preliminary clinical results indicate that this system can measure meconium not apparent to the attending staff; the system can detect changes in the meconium concentration of amniotic fluid; the measurements are confirmed by visual observation at delivery; and changes of meconium concentration seem to correlate with known stressful stimuli. The system therefore provides a new tool from which new variables are obtained, which can greatly enhance clinical research into the pathophysiology of meconium passage and aspiration.