OBJECTIVE: To assess the effects of maternal position on maternal and fetal heart rate and heart rate variability (HRV) in healthy late gestation pregnancies using non-invasive techniques during overnight studies. METHODS: In an observational study of women between 34 and 36 weeks of pregnancy conducted from September 1, 2013, to March 31, 2014, at Columbia University Medical Center, New York, US, maternal and fetal ECG recordings and position monitoring were undertaken through the night in the woman\'s own home. These data were used for time domain analyses of fetal and maternal heart rate and HRV. RESULTS: Forty-two women were recruited to the study which showed that maternal position affected maternal heart rate (MHR), with left side sleeping associated with lower heart rate (left vs right P=0.017, left vs supine P=0.027) and higher overall HRV (left vs right P=0.032). MHR showed significant overnight changes (P=0.032). No significant positional or overnight effects were observed in fetal heart rate patterns. CONCLUSION: This study uniquely incorporated analyses on maternal and fetal physiology and extended the knowledge of effects of maternal overnight sleep position on MHR in the natural sleep environment.