From adult data, it is known that numerous factors, such as age, state of the autonomic nervous system, diurnal rhythms or mean R-R interval rnRR, influence heart rate variability (HRV). The aim of this study was the examination of the influence of gestational age, rnRR, gender and time of day on fetal HRV. The analysis was based on 66 fetal magnetocardiograms (FMCGs) of 22 healthy fetuses between the 16th and 42nd week. FMCGs were recorded for 5 min using a multichannel biomagnetometer. On the basis of the time series of fetal R-R intervals, rnRR as well as the standard deviation sdRR, root mean square of successive differences rrnssdRR and approximate entropy ApEn were calculated. The influence of gestational age, rnRR and gender on sdRR, rrnssdRR and ApEn was determined by regression analysis. The relationship between time of day and HRV was evaluated by visual inspection of scatterplots. The Iogarithmised HRV measures increased significantly with the logarithm of gestational age (regression coefficients: sdRR-1.28, rrnssdRR-1.12, ApEn-1.30) and rnRR (regression coefficients: sdRR -0.008, rrnssdRR -0.011, ApEn -0.012) There was no significant influence of gender. With respect to time of day (between 0800 h and 1800 h), no dependency of the HRV measures was apparent. In summary, when fetal HRV is assessed, it is essential to take gestational age and rnRR into account. In contrast, time of day, with respect to daytime, and gender need not be considered. In future studies, the influence of fetal activity state on HRV should be examined.