SUBJECTS AND METHODS: We investigated the frequency-domain variability and instability in the FHR from 150 uneventful pregnancies (control group), 47 pregnancies with idiopathic IUGR (iIUGR group) and 48 with IUGR associated with severe pre-eclampsia of the mother (PEIUGR group). The FHR was analyzed for the power spectrum and very short-term intermittency (C1alpha), which are used to quantify the instability in FHR.

RESULTS: Compared to the control group, the iIUGR group showed significantly increased low- and high-frequency powers (LFP and HFP, respectively). In contrast, the PEIUGR group showed significantly decreased LFP and LFP/HFP ratio, but significantly increased C1alpha compared to the control and iIUGR groups.

CONCLUSION: The IUGR of an idiopathic cause produces abnormally increased the autonomic modulation of the FHR. In contrast, the IUGR due to pre-eclampsia abnormally decreased the autonomic modulation in the low-frequency range, but increased the instability of the fetal heart rate regulation.