Objective: to assess the effectiveness and acceptability of a pillow-like position modification device to reduce supine sleep during late pregnancy, and to determine the impacts on the severity of sleep-disordered breathing (SDB) and fetal wellbeing. Design: Randomised cross-over study Setting and population: Individuals in the third trimester of pregnancy receiving antenatal care at a tertiary maternity hospital in Australia. Methods: Participants used their own pillow for a control week and an intervention pillow for a week overnight, in randomized order. Sleep position and total sleep time for each night of both weeks was objectively monitored, with a sleep study and fetal heart rate monitoring performed on the last night of each week. Main Outcome Measures: Percentage of sleep time in the supine position, apnoea-hypopnoea index, fetal heart rate decelerations and birthweight. Results: Forty-one individuals were randomized with data collected on 35 participants over 469 nights. There was no difference in percentage of total sleep time in the supine position overnight between the control or intervention pillow week (13.0% [6.1, 25.5] v 16.0% [5.6, 27.2], p = .81), and no difference in the severity of SDB or fetal heart rate decelerations across weeks. However, increased supine sleep was significantly related to a higher apnea-hypopnea index (r s = .37, p = .003), lower birthweight (r s = -.45, p = .007) and lower customised birthweight centile (r s = -.45, p = .006). The proportion of supine sleep each night of the week varied widely both within and across participants, despite awareness of side-sleeping recommendations. Conclusions: The adoption of a pillow designed to discourage supine sleep was not effective in late pregnancy, with women spending an average of one hour per night supine. Supine sleep was associated with SDB and lower birthweight. Alternative devices should be investigated, incorporating lessons learnt from this study to inform trials of supine sleep minimisation in pregnancy. Clinical Trial Registration Number – ACTRN12620000371998 (Australia New Zealand Clinical Trials Registry)