Sleeping Position in Pregnancy

Late intrauterine fetal death (IUFD, or stillbirth) is the death of a baby in the womb occurring at 34 weeks or later in the pregnancy but before labor has begun. Late IUFD is a tragedy that devastates the mother, father, family, and community and has lasting physical and mental health effects on the mother and father similar to the death of a child.[1] About two-thirds of the time, late IUFD is solely a result of poor placenta function,[2] which reduces the amount of oxygen and nutrients that are able to reach the developing baby. Poor placental function plays a secondary role in an additional 14% of late IUFD.[2] In 10% of late IUFD cases, the cause is unexplained even after placental testing and autopsy of the baby in the most specialized of maternity hospitals.[2] In high-income countries like Canada, the top three risk factors for poor placenta function and late IUFD are maternal age over 35, maternal obesity, and maternal smoking.[3] Notice that of these three risk factors, the only one that can be realistically eliminated in the course of a nine-month pregnancy is smoking.

Recently, however, sleeping on the back in late pregnancy has been discovered as a risk factor for late IUFD in addition to other adverse pregnancy outcomes such as delivering a small-for-gestational-age infant.[4–11] Research shows that if all pregnant women avoid sleeping on the back in late pregnancy, the overall late IUFD rate could be reduced by 5.8%.[8] For comparison, if all pregnant women quit smoking, the overall late IUFD rate could be reduced by 5.5%.[3] For a given pregnant woman who usually goes to sleep lying on her back, by going to sleep on her left instead, she can reduce her risk of late IUFD by 61%,[8] whereas a 10% reduction in her pre-pregnancy weight would only reduce her risk of IUFD by 10%.[12] 

It is not yet fully known how sleeping on the back in late pregnancy and late IUFD are linked; however, some evidence shows that it is likely to do with how laying on the back affects blood flow to and from the womb and placenta (see figure, below).[13–21] While we all look symmetrical on the outside, we are not symmetrical on the inside. On the inside, the high-pressure, muscular, and thick-walled blood vessel that carries oxygenated blood from the mother’s heart to the womb (the “aorta”) lies slightly to the left of the spine, and the low-pressure, thin-walled blood vessel that carries deoxygenated blood from the womb back to the mother’s heart (the “inferior vena cava”) lies slightly to the right of the spine. As the pregnancy advances, the developing baby and womb increase in size and weight and they can compress these blood vessels against the bones of the spine when the mother-to-be lies on her back. Compression of these vessels can result in unfavourable changes in the blood flow within the mother-to-be and developing baby,[13] which may be made worse in the setting of poor placental function.

In response to early studies published about this issue,[4,10] we began developing the PrenaBelt™ in 2013. Visit our PrenaBelt Research page to learn more!


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    2. Lou SK, et al. Diagnostic Utility of Pathological Investigations in Late Gestation Stillbirth: A Cohort Study. Pediatr Dev Pathol Off J Soc Pediatr Pathol Paediatr Pathol Soc, 2020.
    3. Flenady V, et al. Major risk factors for stillbirth in high-income countries: a systematic review and meta-analysis. The Lancet, 2011.
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    6. McCowan LME, et al. Going to sleep in the supine position is a modifiable risk factor for late pregnancy stillbirth: Findings from the New Zealand multicentre stillbirth case-control study. PLoS ONE, 2017.
    7. Heazell A, et al. Association between maternal sleep practices and late stillbirth - findings from a stillbirth case-control study. BJOG Int J Obstet Gynaecol, 2018.
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    9. Anderson NH, et al. Association of Supine Going-to-Sleep Position in Late Pregnancy With Reduced Birth Weight. JAMA Netw Open, 2019.
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    11. O’Brien LM, et al. Maternal sleep practices and stillbirth: Findings from an international case-control study. Birth Berkeley Calif, 2019.
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    13. Couper S, et al. The effects of maternal position, in late gestation pregnancy, on placental blood flow and oxygenation: an MRI study. J Physiol, 2021.
    14. Humphries A, et al. The effect of supine positioning on maternal hemodynamics during late pregnancy. J Matern-Fetal Neonatal Med, 2019.
    15. Stone PR, et al. Effect of maternal position on fetal behavioural state and heart rate variability in healthy late gestation pregnancy. J Physiol, 2017.
    16. Higuchi H, et al. Effect of lateral tilt angle on the volume of the abdominal aorta and inferior vena cava in pregnant and nonpregnant women determined by magnetic resonance imaging. Anesthesiology, 2015.
    17. Kienzl D, et al. Risk of inferior vena cava compression syndrome during fetal MRI in the supine position - a retrospective analysis. J Perinat Med, 2014.
    18. De-Giorgio F, et al. Supine hypotensive syndrome as the probable cause of both maternal and fetal death. J Forensic Sci, 2012.
    19. Khatib N, et al. The effect of maternal supine position on umbilical and cerebral blood flow indices. Eur J Obstet Gynecol Reprod Biol, 2014.
    20. Rossi A, et al. Quantitative cardiovascular magnetic resonance in pregnant women: cross-sectional analysis of physiological parameters throughout pregnancy and the impact of the supine position. J Cardiovasc Magn Reson, 2011.
    21. Robertson N, Okano S, Kumar S. Sleep in the Supine Position During Pregnancy is Associated with Fetal Cerebral Redistribution. J Clin Med, 2020.
    Disclaimer: The information provided on this page is for informational purposes only and is not professional medical advice, diagnosis, treatment, or care, nor is it intended to be a substitute therefore.

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