Sleeping on Back or Right Side May Increase Stillbirth Risk

Pauline Anderson

June 15, 2011

June 15, 2011 — Compared with pregnant women who sleep on their left side, those who sleep on their back may have double the risk of delivering a stillborn baby, and those who sleep on their right side also may have an increased risk, according to a new study.

The link between sleep position and stillbirth was independent of other risk factors for late stillbirth, including obesity, and although the association was strongest on the last night before the birth, it was also seen earlier in pregnancy.

"Further studies, ideally with prospectively collected sleep data, are urgently needed to confirm or refute our finding," write the authors, led by Tomasina Stacey, midwifery lecturer and PhD student, Department of Obstetrics and Gynaecology, University of Auckland, New Zealand. Until then, it is too early to recommend a particular sleep position to avoid stillbirth, the authors say.

The case-control study, the first of its kind, was published online June 14 in the British Medical Journal.

The aim of the Auckland Stillbirth Study was to identify potentially modifiable risk factors for late stillbirth, defined as the birth of a baby that died in utero at 28 weeks or more of gestation. The prevalence of late stillbirth among all pregnancies in the Aukland region between July 2006 and June 2009 was 3.09/1000 births.

The analysis included 155 of these women who had experienced a late stillbirth and 310 pregnant control patients who were matched to cases by gestation but went on to deliver a healthy baby. Women with late stillbirth were more likely to be obese, socioeconomically deprived, smoke, and be of high parity compared with the control patients.

Through interviews and questionnaires, researchers gathered demographic data and detailed information on the women's sleep habits and sleep position. They asked about such habits as when the women were going to sleep and when they were waking up before pregnancy and in the last month, week, and night before the stillbirth, or in the case of control patients, the night before the interview.

After adjusting for a range of potential confounders, not going to sleep on the left side on the last night of pregnancy was independently associated with the risk for later stillbirth, with sleeping on the back having the greatest risk. Compared with women who slept on their left side, the adjusted odds ratio (OR) for back sleep on the previous night was 2.54 (95% confidence interval [CI], 1.04 - 6.18; P = .005). The adjusted OR for right-sided sleeping was 1.74 (95% CI, 0.98 - 3.01; P = .005).

Maternal position on going to sleep in the last month was also associated with late stillbirth risk, but none of the individual ORs reached statistical significance.

A theory that might explain the association between sleep position and late stillbirth is that the enlarged uterus of a pregnant woman in a supine position can exert greater pressure on the inferior vena cava and the aorta, resulting in reduced venous return and limited uterine blood flow.

The study also found that getting up to go to the toilet once or less during the last night of pregnancy was significantly associated with late stillbirth risk (adjusted OR, 2.42; 95% CI, 1.46 - 4.00; P = .002).

The researchers used self-reported snoring, in addition to daytime sleepiness measured with the Epworth sleepiness scale, as a proxy indicator for sleep disordered breathing. The study found no association between risk for late stillbirth and snoring either before or during pregnancy. There was also no difference between levels of daytime sleepiness and mean sleepiness score, but sleeping regularly in the daytime during the last month of pregnancy was associated with increased risk.

Among the study's limitations were a possible recall bias and a bias resulting from the length of time between stillbirth and the interview (an average of 25 days compared with control patients, who were asked about sleep practices on the previous night). In addition, the exact timing of fetal death was not always known.

Study Results Inadequate to Launch Sleep Position Campaign

In an accompanying editorial, Lucy C. Chappell, PhD, clinical senior lecturer in maternal and fetal medicine, King's College London, United Kingdom, and Gordon C.S. Smith, MD, PhD, professor and head, Department of Obstetrics and Gynaecology, University of Cambridge, United Kingdom, said the study results are not strong enough to warrant a forceful campaign urging pregnant women to sleep on their left side.

"Although the message for mothers to sleep on their left is probably harmless and may be helpful, this study should be seen as one that only generates a hypothesis that needs validation."

However, if future research supports these findings, advice on sleep position would be “relatively easy to implement,” they said.

Reiterating a limitation raised by the study authors, results of this current retrospective analysis may be explained by bias, wrote Dr. Chappell and Dr. Smith, who noted that the women who had experienced a stillbirth completed questionnaires 25 days after the event. They also pointed out that the study did not include any analysis of cause of stillbirth as an influential variable, making it difficult to assess the biological plausibility of the study's findings.

"A greater association between non-left sided sleep position and stillbirth in fetuses vulnerable to impaired uteroplacental blood flow, such as those with growth restriction, would add weight to the finding," they write.

There is also a strong possibility that part of the association can be explained by reverse causation, said Dr. Chappell and Dr. Smith. "Compromised babies may have reduced movements in the days leading up to the death. Hence, rather than being a cause of stillbirth, the associations between longer sleep and not rising during the night in the week before stillbirth may reflect absent or reduced fetal movements, as a consequence of the baby's death."

The study was supported by Cure Kids, the Nurture Foundation, and the Auckland District Health Board Trust Fund. Ms. Stacey and 2 other authors received support from Cure Kids for the submitted work. The other 3 authors as well as the 2 editorialists, Dr. Chappell and Dr. Smith, have disclosed no relevant financial relationships.

BMJ. Published online June 14, 2011. Full text

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