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Rapid response to:

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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

Rapid Response:

Authors' response to the Rapid Responses to maternal sleep practices and risk of late stillbirth

We totally agree with the accompanying editorial and contributors to
Rapid Response that these novel findings need to be treated with caution
and that it is premature to consider a public health intervention. Indeed
we stated this in the paper and in all our subsequent interactions with
the media.

Dr Froen et al suggests the findings might be due to recall bias.
Recall bias was reduced as far as possible in this study by using a
structured interview and by ensuring that participants were not aware of
the study hypotheses being tested. Sleep position and getting up in the
night have not previously been related to stillbirth, so it is unlikely
that recall bias was present.

Case-control studies are potentially subject to misclassification,
especially as the interviews of mothers of stillbirths were conducted 2-3
weeks after the death. Misclassification reduces the ability to detect a
difference between the cases and controls. However, misclassification is
unlikely to produce a systematic difference between cases and controls,
and thus explain the findings.

Dr Froen et al raises the interesting possibility that the findings
are due to confounding by fetal growth restriction. They argue that the
smaller uterus might result in a reduction in the normal progression in
pregnancy towards the preference for a lateral sleeping position, less
bladder compression, hence not having to get up to the toilet as
frequently, and thus better and longer sleep duration. We have conducted
an additional analysis which adjusts for whether the infant was small for
gestational age (SGA; < 10th customised centile). Model 1 is the
published adjusted odds ratios (1). Model 2 also adjusts for SGA, and the
point estimates and level of significance change very little (Table).

In addition they hypothesise that higher rates of daytime sleep and
more nights with less than 6 hours sleep are indicative of a sick mother
with a complicated pregnancy. The number of mothers of stillbirths with
pre-existing disease were small (unpublished data; previous UTI=6%, pre-
existing hypertension=1%, pre-gestational diabetes=2%), and did not differ
between cases and controls). We have reanalysed the results adjusting for
pre-existing diabetes, polycose >7.7 mmol/L and any pre-existing
medical conditions and again this did not change the associations between
maternal sleep practices and risk of late stillbirth.

We thank Dr Gordon et al for their support and their response to the
comments from Dr Starr. Dr Starr criticised the use of self-reported
maternal sleeping position. However, when women were asked about sleep
position, we attempted to validate how they knew what position they went
to sleep in. We received comments such as "I always faced away from the
door", "I liked to sleep facing the wall", "we had to change which side of
the bed we slept because the other side became more comfortable". If
women could not recall or were unsure they were classified as "other" and
this did not differ between cases and controls.

Observational studies show associations between the factor under
investigation and the outcome. Of course such studies cannot prove
causation, nor can they provide direct proof of reduced uterine blood
flow, as noted by Dr Alouuini. However, this research is hypothesis
generating, needs to be replicated, and the putative mechanisms
investigated. Only at that stage should a public health intervention be
considered, and of course it will be necessary to deliver such an
intervention in a culturally effective manner without causing guilt and
anxiety.

However, the excitement is that if this association is confirmed, and
it is causally associated with late stillbirth, then there is the
potential to reduce late stillbirth by over a third.

Reference
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; d3403
doi:10.1136/bmj.d3403

Competing interests: No competing interests

27 June 2011
Ed A Mitchell
Professor of Child Health Research
Stacey T, Thompson JMD, Ekeroma AJ, Zuccollo JM, McCowan LME
University of Auckland