Background: Spectral analysis of fetal heart rate variability is promising for assessing fetal condition. Before using spectral analysis for fetal monitoring it has to be determined whether there should be a correction for gestational age or behavioural state. Aims: Compare spectral values of heart rate variability between near term and post term fetuses during active and quiet sleep. Study design: Case-control. Cases had a gestational age of = 42 weeks; controls were 36 to 37 weeks. Fetuses were matched for birth weight percentile. Subjects: STAN® registrations from healthy fetuses. For each fetus one 5-minute segment was selected during active and one during quiet sleep. Outcome measures: Absolute and normalized low (0.04-0.15 Hz) and high frequency power (0.4-1.5 Hz) of heart rate variability. Results: Twenty fetuses were included. No significant differences were found between cases and controls in absolute (481 and 429 respectively, P = 0.88) or normalized low (0.78 and 0.80 respectively, P = 0.50) or absolute (41 and 21 respectively, P = 0.23) or normalized high frequency power (0.08 and 0.07 respectively, P = 0.20) during active state. During rest, normalized low frequency power was lower (0.58 and 0.69 respectively, P = 0.03) and absolute (16 and 10 respectively, P = 0.04) and normalized high frequency power were higher (0.21 and 0.14 respectively, P = 0.01) in cases compared to controls. Absolute and normalized low frequency power were higher during active state compared to rest in both groups (all P values <0.05). Conclusions: We found sympathetic predominance during active state in fetuses around term. Post term parasympathetic modulation during rest was increased compared to near term. © 2009 Elsevier Ireland Ltd. All rights reserved.