A computer method for quantification of fetal heart rate (FHR) variability from fetal magnetocardiography during pregnancy and from direct fetal electrocardiography during labor is presented. It is based on statistical analysis of the QRS interval sequences. Beat-to-beat variation is characterized by a differential index (DI) and long-term variation by an interval index (II). The effect of the sample time on the DI is minimal, and hence the DI can be calculated from rather short samples. The II is more sensitive to FHR trends and should be calculated from longer samples, but between the periodic changes, accelerations, and decelerations. Variable amounts of detection pulses are lost in both methods. The DI is sensitive to the missing intervals; no analysis result should be accepted if the number of lost intervals exceeds 10 per cent. The II is less sensitive to the number of missing intervals. The means and standard deviations of the variability indices for eight fetuses during pregnancy and for five fetuses during labor are presented.