Intrauterine growth restriction is a risk factor for the development of the hypertension in adult life. The purpose of this study was to examine the differences in abnormality in instablity and autonomic modulation of fetal heart rate (FHR) between the growth restricted fetuses caused by the idiopathic cause and the maternal preeclampsia. For this purpose, FHRs that were recorded in the third trimester from 150 uneventful pregnancies (control group), 47 pregnancies with idiopathic IUGR (iIUGR group), and 48 pregnancies with intrauterine growth restriction associated with severe preeclampsia of the mother (PEIUGR group) were used. For each group, The very short-term intermittency (C1a) and power spectrum were calculated to quantify the instability and autonomic modulation of FHR. When compared to control group, the iIUGR group showed no significant difference in C1a and significantly increased low- and high-frequency power (P<0.05 and P<0.005, respectively). However, the PEIUGR group showed significantly increased C1a (P<0.0005) and decreased low- and high-frequency power (P<0.0005 and P<0.05, respectively). These results indicate that when intratuerine growth restriction is caused by maternal preeclampsia, unlike when the growth restriction is of idiopathic cause, the FHR of the growth restricted fetuses becomes abnormally unstable and less modulated by cardiac autonomic nervous modulation.