Background Fetal behavioural states (FBS) are measures of fetal wellbeing. Maternal position affects FBS with supine being associated with an increased likelihood of fetal quiescence consistent with the human fetus adapting to a lower oxygen consuming state. A number of studies now confirm the association of sleep position with risk of late intrauterine death. We designed this study to observe the effects of maternal sleep positions overnight in healthy late gestation pregnancy. Method Twenty nine healthy women had continuous fetal ECG recordings overnight. Two blinded observers, assigned fetal states in 5 minute blocks. Measures of fetal heart rate variability (FHRV) were calculated from ECG beat to beat data. Maternal position was determined from infrared video recording. Results Compared to state 2F (active sleep), 4F (active awake‐high activity) occurred almost exclusively when the mother was in a left or right lateral position. State 1F (quiet sleep) was more common when mother was supine (OR 1.30, 95%CI, 1.11‐1.52) and less common on maternal right side with the left being referent position (OR 0.81, 95%CI, 0.70‐0.93). State 4F was more common between 2100 and 0100 than between 0100 and 0700 (OR 2 2.83, 95%CI, 2.32‐3.47). In each fetal state, maternal position had significant effects on fetal heart rate (FHR) and measures of FHRV. Conclusion In healthy late gestation pregnancy, maternal sleep position affects FBS and heart rate variability. These effects are likely fetal adaptations to positions which may produce a mild hypoxic stress. This article is protected by copyright. All rights reserved