Wednesday June 15 2011
The researchers call for further studies into their findings
Widespread media coverage has been given to a study on the risk of stillbirth and the sleeping position of the mother. “Mums-to-be should sleep on their left side”, reported The Mirror. The Daily Mail said that “women who sleep on their right side or back during the late stages of pregnancy could be at higher risk of stillbirth.”
This news story comes from a study that compared sleeping position and other sleep behaviours in 155 women who had stillborn babies with 310 women who had live births. From these women, the researchers calculated that in high-income countries, stillbirth will occur at a rate of about 3.09 per 1,000 births. If women slept on their right side or their back in late pregnancy this risk was 3.93 per 1,000, compared to 1.96 per 1,000 if they slept on their left side.
These observations are plausible, but this small study has several limitations and can only show an association between sleeping position and stillbirth. It cannot conclusively prove that a woman’s sleeping position affects the risk of stillbirth. This study alone does not provide sufficient evidence to promote an ideal sleep position for pregnant women. This is confirmed by the editorial that accompanies the study, which states: “A forceful campaign urging pregnant women to sleep on their left side is not yet warranted. Further research is needed before the link between maternal sleep position and risk of stillbirth can be regarded as strongly supported.”
It is also important to note that the risk of stillbirth is generally low, regardless of sleeping position.
Pregnant women should speak to their midwives or GPs if they are concerned.
Where did the story come from?
The study was carried out by researchers from the University of Auckand and the Wellington Medical School in New Zealand. It was published in the peer-reviewed British Medical Journal.
The research was funded by Cure Kids, the Nurture Foundation and the Auckland District Health Board Trust Fund.
The Independent and The Guardian both reported that the results were preliminary and that the absolute risks of stillbirth were low. However, many newspaper headlines implied that a causal link between sleep position and stillbirth has been conclusively found, but this was not the case. This research has numerous limitations, which are discussed below.
What kind of research was this?
This case-control study aimed to determine whether snoring, sleep position and other sleep practices in pregnant women were associated with the risk of late stillbirth. The research was carried out in a population of pregnant women in Auckland. The researchers compared the circumstances of women who gave birth to a stillborn baby at or after 28 weeks of pregnancy (cases) and women who were pregnant at the same time and went on to have a live birth (controls).
Women who gave birth to a stillborn baby were identified from maternity units in the Auckland region. The controls were selected from the districts’ pregnancy registration lists, and two controls in the same week of pregnancy were matched to each case. The study did not include women who were pregnant with more than one baby, those whose baby had a congenital abnormality, or those who were registered outside Auckland.
The researchers say that previous studies have found that sleeping on the back is associated with a disruption in normal breathing patterns. They also say that lying flat on the back during late pregnancy can reduce the mother’s cardiac output (the amount of blood pumped out from the heart with each heartbeat). This is because the major veins in the body that return blood to the heart (the vena cavae) are positioned just to the right side of the centre of the body. As such, it is possible that in heavily pregnant women, the pressure of the baby when lying flat or just to the right could interrupt the return of blood to the heart. This could reduce the mother’s cardiac output and, in turn, affect blood supply to the uterus and placenta.
For this reason, women having a caesarean section are normally positioned lying tilted towards their left to take pressure off the vena cavae. However, there has been no research into whether maternal sleep position and other sleep-related factors affect the risk of late stillbirth.
The researchers’ theory was that both sleep-disordered breathing and lying on the back would be associated with increased risk of stillbirth late in pregnancy.
What did the research involve?
Researchers interviewed women during the few weeks after stillbirth, asking them about their sleeping habits and factors that can affect the risk of stillbirth. They used self-reported snoring and daytime sleepiness as substitutes for sleep-disordered breathing, as there is no validated tool for assessing sleep-disordered breathing during pregnancy.
The women were also asked about their sleep position (left side, right side, back or other) before the pregnancy, during the last month, during the last week and during the last night of pregnancy. They were also asked about their daytime sleep patterns during the last month, and how often they got up during the night. Data were also collected on factors that previous studies have suggested are associated with increased risk of stillbirth. These included maternal age, ethnicity, number of previous full-term pregnancies, smoking status, body mass index and level of social deprivation.
The interviews of the cases occurred on average 25 days after the date of the stillbirth. All factors were analysed independently to detect any association with stillbirth. The women in the control group were asked about their previous night's sleep patterns and other factors at the same point in their pregnancy that their matched case experienced the stillbirth. For example, if a woman experienced a stillbirth at 30 weeks, her matched control would be interviewed when she was 30 weeks' pregnant (i.e. before she had her baby).
The researchers then carried out analyses on maternal factors that were associated with stillbirth, taking into account any that had been identified as increasing the risk of stillbirth.
What were the basic results?
The main findings of the research were:
- Compared to sleeping on the left side, sleeping on the right side was not significantly associated with increased risk of stillbirth.
- Compared to sleeping on the left side, sleeping on the back and in other positions was associated with an increased risk (2.54 and 2.32 times more likely to have a stillbirth respectively).
- Women who got up to go to the toilet once or not at all during the last night of pregnancy were 2.42 times more likely to experience stillbirth as those who got up more.
- Women who reported regularly sleeping during the day in the last month of pregnancy were 2.04 times more likely to experience stillbirth as those who didn’t.
- Duration of sleep at night during the last night of pregnancy had no significant effect on risk.
- There was no observed association between snoring and risk of stillbirth.
The researchers point out that the absolute risk of late-term stillbirth in high-income countries is low (3.09 per 1,000 births in the study population), and non-left-side sleeping only slightly increased this risk (to approximately 3.93 per 1,000 births).
How did the researchers interpret the results?
The researchers concluded that their preliminary study has produced findings that “require urgent confirmation in further studies”. They say that they have identified a potentially modifiable risk factor for stillbirth, but acknowledge both the strengths and limitations of their study.
This study found that not sleeping on the left side during the last night of pregnancy was associated with a higher risk of stillbirth.
The researchers used an appropriate study design to investigate their theory. A case control study such as this is useful for looking at outcomes that are relatively rare, such as stillbirth. The participants were also carefully selected and well matched, which improves the chances that these findings can be applied to the wider population of pregnant women. However, the study had several limitations that must be considered when interpreting its findings, and which the researchers acknowledge:
- The results may be subject to several biases. For example, the women may not have accurately recalled their sleeping position, which is difficult to confirm. As the researchers say, there is currently no validated method for assessing sleep-disordered breathing or sleep pattern during pregnancy. This limitation is partly alleviated by the fact that most participants used reference points for their sleep position, such as “I always faced away from the door” or “I slept facing my husband”.
- It’s also possible that the women who had stillborn babies may have misremembered the events leading up to delivery as they sought to find a reason for the trauma they had been through.
- There was also a time gap between when the cases and controls were asked to report their sleeping pattern. Cases were asked about sleep position on average 25 days after their last night of pregnancy, whereas controls were asked to report on the previous night’s sleep position.
- It is also possible that the observed association is an example of reverse causality. In other words, this study cannot determine whether the association between sleeping position and stillbirth is due to sleep position increasing the risk of stillbirth, or stillbirth resulting in changed sleep patterns. The last night of pregnancy before stillbirth was examined but, as the researchers point out, the last night of pregnancy may not have been the night before the foetus died, which may have died before this time. This may have altered the results. For example, if the foetus had already died, absent foetal movements could have influenced the mother’s choice of sleep position, or reduced the number of times she got up during the night because she had less disturbance to her sleep. Therefore, sleep position and reduced frequency of getting up during the night may have been a result of foetal death, rather than a risk factor for it.
- Finally, it is possible that an unknown factor is associated with both sleep behaviour and stillbirth and which accounts for the observed relationship.
The observations of this study are plausible. However, the study was small and can only detect associations, rather than conclusively proving that sleeping position affects the risk of stillbirth. This study alone does not provide sufficient evidence to support widespread campaigns promoting an ideal sleep position for pregnant women. This is confirmed by the editorial that accompanies the study’s findings, which says: “A forceful campaign urging pregnant women to sleep on their left side is not yet warranted. Further research is needed before the link between maternal sleep position and risk of stillbirth can be regarded as strongly supported.”
It is also important to note that the risk of stillbirth is quite low, regardless of sleeping position. The researchers calculated that in a high-income country, stillbirth occurs at a rate of 3.09 per 1,000 births. Sleeping in any position other than the left side (such as on the back or right side) resulted in a risk of 3.93 per 1,000 births, while sleeping on the left side would result in a risk of 1.96 per 1,000.
Though this study cannot conclusively prove that sleep pattern affects the risk of stillbirth, these findings require further investigation. Future research should attempt to address some of the potential limitations of this study. Further research would also benefit from a validated method of measuring sleep-disordered breathing in pregnant women.
Pregnant women should speak to their midwives or GPs if they are concerned.