Oxygen levels and lying on your back when pregnant

Going to sleep lying flat on the back (supine position) has been linked to a higher rate of stillbirth between 28 weeks of pregnancy and the start of labour, with the chance rising closer to the end of pregnancy (McCowan, et al., 2017). Because of this, many countries now issue public recommendations about going to sleep in a side lying position in late pregnancy. The McCowan study also showed a relationship between the baby being under the 10th percentile for weight at birth and stillbirth.

Recent research from New Zealand set out to explore inter-relationships between body position and fetal size by looking at blood flow and oxygen transfer through the placenta (Jani, et al., 2023). It makes for interesting reading.

How was the research done?

39 pregnant women agreed to participate in the study. They were between 34 and 38 weeks of gestation, each with a single fetus with normal fetal anatomy on scan, and with a pre-pregnancy body mass index between 18 and 30 kg/m². Women with medical conditions (like gestational diabetes), or with variations in placental structure or location (like a bilobed placenta or placenta previa) were not eligible to enrol. Two specific subsets of women were asked to join the study – those with “appropriate for gestational age” (AGA) sized fetuses and those with “small for gestational age” (SGA) fetuses. To be included in the small for gestational age category, the fetus needed to be under the 3rd percentile on ultrasound on a customised growth chart, or under the 10th percentile with abnormal blood flow in the mother’s uterine artery or the fetal umbilical or middle cerebral artery. Women with fetuses over the 95th percentile were excluded from the study.

All women then had an MRI scan in two positions: while lying on their left hand side and while lying on their back. Which one was used first was decided by a random number generator. Particular focus in the MRI images was on the placenta, the mother’s blood vessels leading to the placenta (the internal iliac arteries), and the umbilical vein carrying blood from the placenta to the fetus. This meant researchers could determine a whole pile of measures relating to blood flow and oxygen levels, including fetal oxygen saturation, the delivery of oxygen across the placenta (placental flux), and the delivery of oxygen from the placenta to the fetus (delivery flux).

What did they find?

27 of the women were in the AGA category, and 12 in the SGA category – with five having a fetus with an estimated fetal weight of under the 3rd percentile. Most women were white and pregnant for the first time. All give birth to a live baby. Two of the babies born to women in the AGA category were at or under the 10th percentile at birth. Actual birth weights for babies born to women in the SGA category ranged from 1890 g (less than the 1st percentile) to 2890 g (15th percentile), with the authors not providing information about the number who were not under the 3rd, or 10th percentiles, at birth. This lack of alignment between estimated and actual weight is consistent with research on the accuracy of ultrasound estimates.

Fetal oxygen saturation was significantly lower in the supine position (a fall of 3.8%) compared with the left lying position, as was placental flux. There was a large drop in delivery flux in this position as well. Within the range of BMIs of women included in this study, BMI made no difference to blood flow to the placenta.

Fetal oxygen saturation, placental flux, and delivery flux were all significantly lower in women in the SGA category than those in the AGA category. Supine positioning led to a relative reduction in oxygen saturation of 12% in the SGA group and 18% of the AGA group. Supine positioning in the SGA group reduced placental flux by 18% and delivery flux by 41% compared to left lying in the AGA group.

In other words: lying flat on the back reduced oxygen transfer from the mother to the fetus, and this was more of an issue for fetuses that were already small.

Important things to bear in mind

In the long term, all these babies did OK. One baby in the AGA group was admitted to the nursery for a short period (with meconium aspiration), and another from the SGA group (with transient breathing difficulties). Even when oxygen saturation levels fell, they remained in the range considered normal for a fetus. This study doesn’t tell us what happens during sleep, as the women remained awake during the study, and there are other physiological changes that happen with sleep. It also provides no information about upright positions or being active (and we’ll never manage to get an MRI machine to do a study like that because it isn’t how they work). The study provides no information about whether what they found was true for women with smaller or larger BMIs. There’s less information about women who weren’t white (who are more likely to experience either SGA or stillbirth) and they aren’t separated out in the findings so we can’t tell what the impact of position was for them.

What’s the message here?

The study authors said:

The study has provided further support for educating all pregnant women, especially those with pregnancies affected by fetal growth restriction, to avoid resting and going to sleep in the supine position to reduce their risk of late stillbirth.
p. 17

There’s a fine balance to be achieved here – between information provision to support women to make decisions on the one hand, and scaring women while reinforcing messages that women’s bodies are not to be trusted as a life support system for a fetus on the other. I think the study helps researchers to better understand the way MRI might be used to provide useful clinical information and to add to our knowledge about fetal physiology and pathophysiology. I’m not sure their findings are robust enough to guide advice about side-lying, but if that’s easy for you and doesn’t lead to anxious thoughts about fetal wellbeing – then there’s no evidence to suggest side lying in late pregnancy is a problem. If your fetus is considered to be small, then side lying while resting or heading to sleep might give you more of a sense of control over the situation.

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References

Jani, D., Clark, A., Couper, S., Thompson, J. M. D., David, A. L., Melbourne, A., Mirjalili, A., Lydon, A. M., & Stone, P. R. (2023, Jul 19). The effect of maternal position on placental blood flow and fetoplacental oxygenation in late gestation fetal growth restriction: a magnetic resonance imaging study. Journal of Physiology, in press. https://doi.org/10.1113/JP284269

McCowan, L. M. E., Thompson, J. M. D., Cronin, R. S., Li, M., Stacey, T., Stone, P. R., Lawton, B. A., Ekeroma, A. J., & Mitchell, E. A. (2017). 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, 12(6), e0179396. https://doi.org/10.1371/journal.pone.0179396