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Association between maternal sleep practices and risk of late stillbirth: a case-control study

BMJ 2011; 342 doi: https://doi.org/10.1136/bmj.d3403 (Published 14 June 2011) Cite this as: BMJ 2011;342:d3403

No need to worry about sleeping position in pregnancy - quite yet

J. Frederik Froen 1), Joanne Cacciatore 2), Ruth C. Fretts 3) and
Vicki Flenady 4)

1) Norwegian Institute of Public Health, Oslo, Norway, 2) Arizona
State University, Phoenix, AZ, USA, 3) Harvard Vanguards Medical
Associates, Wellesley, MA, USA, 4) Mater Medical Research Institute and
Queensland University, Brisbane, QLD, Australia

We congratulate Tomasina Stacey and her colleagues for their
exploration of sleeping position in stillbirth(1). Studies such as these
are important in the formation of new hypotheses. Yet, when repeated
measures lead to an unexpected finding with uncertain biological
pathways(1), we agree with other critiques which call for prudence and
circumspect in the interpretation of observational studies(2).

Recall bias is always a limitation when cases are interviewed weeks
after the event while controls are interviewed during pregnancy, as their
usual sleeping position may affect the responses of cases more than
controls. Thus, controls would seem to change preferences, while cases
less so - as is seen in this study(1). However, our main concern is the
probability of a reversed causation. There was no difference in sleeping
positions before pregnancy, and thus no "high risk sleepers" by habit.
This should lead us to consider that something about their pregnancy
shaped their sleeping position, while their habitual sleeping position
probably did not shape their pregnancy (nor its outcome).

As the baby grows, the potential for pressure against the vena cava
in a supine sleeping position increases, as probably do all other known
and unknown mechanisms that make women gradually prefer a (left?) lateral
tilt in late pregnancy. What would reduce normal progression toward the
preference of a more lateral tilt(1), reduce the need to go to the
bathroom at night(1), and thus increase undisturbed sleep for more than 8
hours(1)? A smaller belly. An increased proportion of growth restricted
babies with oligohydramnios among cases would potentially provide exactly
the results found in this study, including the appealing finding of
greater differences in sleeping position as pregnancy progresses. Indeed,
a large proportion of stillborn babies are severely growth restricted(3),
and their progress toward death is a gradual one, not a single sudden
event(4). What would explain higher rates of daytime sleep(1) and more
nights with less than 6 hours sleep(1)? A sick mom with a complicated
pregnancy. Again, not a rare finding in stillbirth(3;4).

In the MOMstudy(5), a population of 2109 women affected by stillbirth
and 1515 live born controls (both >= 28 weeks) responded to a
questionnaire regarding sleeping position. Sleeping position was strongly
correlated with gestational age, birthweight, and small vs. appropriate
for gestation. There was no difference between cases and controls when
stratified by fetal size. Mothers in the control group reported that they
slept "Exclusively on the side" the last four weeks of pregnancy in 36%
(n=124) if their baby weighed < 2500 grams at birth, compared with 54%
(n=39) for birthweights >=4500 grams. The equivalent numbers for cases
were 36% (n=1382) and 58% (n=42), respectively. We found no differences in
right or left tilt preference between cases and controls the last four
weeks of pregnancy, nor any change in sleeping position the last night
mothers felt fetal movements prior to stillbirth. We hope Stacey and
colleagues are able to reanalyze and adjust their findings for fetal size,
as we hypothesize that their significant findings would disappear.

Could sleeping position be "the final drop" for a moribund pregnancy?
We feel that it is premature both to exclude and to conclude that a
potentially modifiable risk factor has been discovered. Promulgating a
public health message of altered sleeping position may have little effect
other than adding to the burden of self-blame, shame, and guilt on
grieving mothers' shoulders(6) - the possibility that any of them slept
"on the wrong side" is yet unproven. There are, however, practices for
women who are pregnant or planning to have a baby that have a strong
evidence base and need to be more actively disseminated including:
reducing overweight and smoking cessation(3;4). Also, a pregnant woman
should trust her instincts if she feels her baby's movements have changed
significantly, and she should contact her health care provider
immediately(4;7).

Reference List

(1) Stacey T, Thompson JMD, Mitchell EA, Ekeroma AJ, Zuccollo JM,
McCowan LME. Association between maternal sleep practices and risk of late
stillbirth: a case-control study. BMJ 2011; 342.

(2) Chappell LC, Smith GCS. Should pregnant women sleep on their
left? BMJ 2011; 342.

(3) Flenady V, Koopmans L, Middleton P, Froen JF, Smith GC, Gibbons K
et al. Major risk factors for stillbirth in high-income countries: A
systematic review and meta-analysis. Lancet 2011; 377(9774):1331-1340.

(4) Flenady V, Middleton P, Smith GCS, Duke W, Erwich JJ, Khong TY et
al. Stillbirths: The way forward in high-income countries. Lancet 2011;
377(9778):1703-1717.

(5) Cacciatore J, Radestad I, Froen JF. Effects of contact with
stillborn babies on maternal anxiety and depression. Birth 2008; 35(4):313
-320.

(6) Froen JF, Cacciatore J, McClure EM, Kuti O, Jokhio AH, Islam M et
al. Stillbirths: Why they matter. Lancet 2011; 377(9774):1353-1366.

(7) International Stillbirth Alliance. ISA Position Statement: Fetal
Movement Monitoring.
http://www.stillbirthalliance.org/doc/DFM_Statement.pdf ; Accessed 2011
June 21.

Competing interests: No competing interests

23 June 2011
J. Frederik Froen
Dept. Director, MD, PhD
Joanne Cacciatore, Ruth C. Fretts and Vicki Flenady
Division of Epidemiology, Norwegian Institute of Public Health