By Camille Hoffman, MD, MSc

Associate Professor, Maternal Fetal Medicine, University of Colorado Departments of Obstetrics and Gynecology and Psychiatry, Wheat Ridge, CO

Dr. Hoffman reports no financial relationships relevant to this field of study.

SYNOPSIS: Questions regarding sleep position safety are often posed for any OB/GYN or midwife practicing obstetrics.

SOURCES: Silver RM, Hunter S, Reddy UM, et al. Prospective evaluation of maternal sleep position through 30 weeks of gestation and adverse pregnancy outcomes. Obstet Gynecol 2019;134:667-676.

Anderson NH, Gordon A, Li M, et al. Association of supine going-to-sleep position in late pregnancy with reduced birth weight: A secondary analysis of individual participant data meta-analysis. JAMA Netw Open 2019;2:e1912614.

In the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-be (nuMoM2b), Silver et al sought to prospectively examine the relationship between maternal sleep position and subsequent adverse pregnancy outcomes in a U.S. population. The authors used an existing, prospectively collected dataset of 8,706 nulliparous women who completed information on their sleep at two pregnancy time points (first and about the third trimester).

A composite primary outcome included stillbirth, small for gestational age (SGA), and hypertensive disorders of pregnancy. In addition to a sleep questionnaire - which was thorough regarding sleep positions - a subset of 2,474 women completed a “sleep-disordered breathing sub-study.”

Although the primary composite outcome occurred in 22% of the study population (1,903 pregnancies), there was no association between reported sleep position in the non-left lateral or supine position at either the first visit (6 weeks 0 days to 13 weeks 6 days) or the third visit (22 weeks 0 days to 29 weeks 6 days). There were 13 stillbirths, and all occurred in the setting of SGA, hypertensive disorders, or both. Most surprisingly, there was a protective effect for stillbirth when women reported sleeping in the non-left lateral or supine position at the third trimester visit (adjusted odds ratio [aOR] 0.27; 95% confidence interval [CI], 0.09-0.75). In the subgroup, which had an additional sleep-disordered breathing study, women who slept ≥ 50% of the time in a supine position were no more or less likely than women who slept in the supine position for < 50% of the time to experience the primary composite outcome.

In contrast, the study by Anderson et al examined the association between supine position (when going to sleep) and lower birthweight and birthweight centiles in pregnant women > 28 weeks. The study population included “selected women from the control group … from the Collaborative Individual Participant Data Meta-analysis of Sleep and Stillbirth study population.” Translation: These were retrospective case-control study “control group” data of 1,760 women who did NOT have stillbirths (cases) from New Zealand, Australia, and the United Kingdom.

The authors cite a significant difference in birth weight based on maternal position when falling asleep. The mean birth weight of infants born to women who reported going to sleep in the supine position was 3,410 g
(± 112 g); the mean birth weight of infants born to women who reported going to sleep in a “non-supine” position was 3,554 g
(± 98 g). This difference was statistically significant (P = 0.009), although the adjusted mean difference was only 144 g and the adjusted customized percentile difference was 40.7 vs. 49.7 percentiles. The authors also reported a threefold increase in SGA birth weight (aOR 3.23; 95% CI, 1.37-7.59) in the supine sleeping group compared with the non-supine sleeping group. The conclusion of this study is that a supine position in pregnancy > 28 weeks was independently associated with reduced birth weight and birth weight percentile. Again, these data come from a retrospective case-control study cohort investigating sleep and stillbirth, and these authors have already published the significantly increased risk of stillbirth based on going to sleep in a supine position. This contrasts with the prospectively collected, and, therefore, presumably less biased, data published in the Silver article.

In this study, more than 90% of the cohort slept in a non-supine position and only ~5% reported going to sleep in a supine position. In the Silver study, 47% reported going to sleep in either a right or left lateral position and 18% (at the third trimester visit) reported going to sleep in a supine position. This also illustrates the bias likely already imposed on participants in the Anderson study, as very few of them slept in a supine position overall.

COMMENTARY

Worries about sleep, stillbirth, and fetal growth occupy some of the Q&A that occurs during routine prenatal care. Popular culture tells women that sleeping on their backs is dangerous to the baby’s oxygenation, and women (along with obstetric providers) long to understand the relationship between sleep position and fetal outcome. In previous studies, maternal sleep position has been associated with stillbirth and fetal growth restriction along with other adverse pregnancy outcomes.^1-4 Campaigns already exist that sternly caution women to avoid sleeping in a supine position.⁵ In a recent study also authored by Anderson, the researchers concluded that avoiding the supine going-to-sleep position would reduce late stillbirth by 6%,⁶ which is very modest and potentially confounded by other risk factors for both poor sleep and stillbirth (obesity, other comorbidities). In contrast, anxiety over the “right” position in which to sleep, disrupted sleep resulting from worry over not doing it correctly, compromise of sleep quality and duration because of trying to stay on one’s left side, and the self-blame that often comes along with an adverse pregnancy outcome also deserve attention.⁷

Physiologically, it makes sense to recommend sleeping on the left side to decompress the inferior vena cava and aorta, and improve overall blood flow to the uteroplacental unit. In addition, women often become symptomatic in the supine position in later gestation. However, aside from the prospective study highlighted earlier, the existing data are subject to recall bias given that they are primarily retrospective and come from interviews/questionnaires conducted in populations affected by stillbirth or other adverse pregnancy outcomes. Consideration of a woman’s guilt and doubt after a poor pregnancy outcome is important to realize in these retrospective studies.

Women in the Silver study all were nulliparous, while 43% of women in the Anderson study were nulliparous. Information on prior pregnancy outcomes in the multiparous participants (i.e., factors that would motivate a patient to be a control in this type of study) was not reported. In the Silver study, lateral vs. supine sleep positioning was not associated with the composite outcome. There were only a small number of stillbirths and all were associated with other adverse pregnancy outcomes. The authors fairly acknowledged that sleep was assessed only through 30 weeks gestation and that correlations between reported sleep position and objectively measured sleep position (in the subgroup) were modest to strong.

On the other hand, in the Anderson study, supine going-to-sleep position was associated with “significantly” lower birth weight (albeit not clinically significant at 3,410 g vs. 3,554 g) BUT there was increased odds of an SGA infant in the supine sleeping group. That being said, only 3.2% of the total study population reported going to sleep in the supine position and, therefore, odds ratios may be distorted by such a large difference in comparison groups.

At the end of the day, pregnancy pillow companies will continue to maintain a market, and campaigns begun by bereaved parents will continue to exist. As a community caring for pregnant women, we are willing to go to great lengths to prevent any stillbirths. Yet, in a large prospective cohort, supine or non-left-sided sleep up to 30 weeks gestation was not associated with adverse pregnancy outcomes. Based on these data, I believe that we can reassure the anxious patient who is less than 30 weeks gestation and woke up in a supine position that she has done no harm to her baby by sleeping in this position. I have started to try and address the anxiety that results from this supposed sleep transgression and encourage women to sleep in whatever position they feel most comfortable prior to 30 weeks.

REFERENCES

1. Stacey T, Thompson JM, Mitchell EA, et al. Association between maternal sleep practices and risk of late stillbirth: A case-control study. BMJ 2011;342:d3404.
2. Gordon A, Raynes-Greenow C, Bond D, et al. Sleep position, fetal growth restriction, and late-pregnancy stillbirth: The Sydney stillbirth study. Obstet Gynecol 2015;125:347-355.
3. Heazell A, Li M, Budd J, et al. Association between maternal sleep practices and late stillbirth - findings from a stillbirth case-control study. BJOG 2018;125:254-262.
4. McCowan LME, Thompson JMD, Cronin RS, et al. Going to sleep in the supine position is a modifiable risk factor for late pregnancy stillbirth: Findings from the New Zealand multicenter stillbirth case-control study. PLoS One 2017;12:e0179396.
5. Tommy’s sleep on side - a pregnancy campaign. Available at: www.tommys.org/pregnancy-information/sleep-side-pregnancy-campaign. Accessed Oct. 19, 2019.
6. Cronin RS, Li M, Thompson JMD, et al. An individual participant data meta-analysis of going-to-sleep position, interactions with fetal vulnerability, and the risk of late stillbirth. EClinicalMedicine 2019;10:49-57.
7. Silver RM. Maternal going to sleep position and late stillbirth: Time to act but with care. EClinicalMedicine 2019;10:6-7.