By means of a new averaging system, fetal electrocardiograms (ECGs) were obtained simultaneously from three orthogonal leads on the maternal abdomen. The duration of the QRS complex was measured in 421 pregnant women with uncomplicated pregnancies chosen at random between 17 and 41 weeks gestation. Normal standards were established and a close relation was found between QRS duration and the maturity of the fetus. A highly significant relation was observed in 78 patients between the birth weight and the QRS duration. Estimates of birth weight from QRS interval have a standard deviation of 302 g and this may allow the prediction of birth weight with reasonable accuracy. Information on the properties of the fetal ventricular conduction system can be derived from the data presented.