Published by Oxford University Press on behalf of the International Epidemiological Association
© The Author 2005; all rights reserved. Advance Access publication 20 April 2005
International Journal of Epidemiology 2005;34:874–887
Infant sleeping position and the sudden
infant death syndrome: systematic review of
observational studies and historical review of
recommendations from 1940 to 2002
Ruth Gilbert,1* Georgia Salanti,2Melissa Harden1and Sarah See1,3
4 April 2005
Background Before the early 1990s, parents were advised to place infants to sleep on their
front contrary to evidence from clinical research.
We systematically reviewed associations between infant sleeping positions and
sudden infant death syndrome (SIDS), explored sources of heterogeneity, and
compared findings with published recommendations.
By 1970, there was a statistically significantly increased risk of SIDS for front
sleeping compared with back (pooled odds ratio (OR) 2.93; 95% confidence
interval (CI) 1.15, 7.47), and by 1986, for front compared with other positions
(five studies, pooled OR 3.00; 1.69–5.31). The OR for front vs the back position
was reduced as the prevalence of the front position in controls increased. The
pooled OR for studies conducted before advice changed to avoid front sleeping
was 2.95 (95% CI 1.69–5.15), and after was 6.91 (4.63–10.32). Sleeping on the
front was recommended in books between 1943 and 1988 based on extrapolation
from untested theory
Conclusions Advice to put infants to sleep on the front for nearly a half century was contrary
to evidence available from 1970 that this was likely to be harmful. Systematic
review of preventable risk factors for SIDS from 1970 would have led to earlier
recognition of the risks of sleeping on the front and might have prevented over
10 000 infant deaths in the UK and at least 50 000 in Europe, the USA, and
Australasia. Attenuation of the observed harm with increased adoption of the
front position probably reflects a ‘healthy adopter’ phenomenon in that families
at low risk of SIDS were more likely to adhere to prevailing health advice. This
phenomenon is likely to be a general problem in the use of observational studies
for assessing the safety of health promotion.
Sudden infant death, review, meta-analysis
1Centre for Evidence-based Child Health, Institute of Child Health, London, UK.
2MRC Biostatistics Unit, Institute of Public Health, Forvie Site, Robinson
Way, Cambridge, UK.
3Present address: Waltham Forest Primary Care Trust (PCT),
* Corresponding author. Centre for Evidence-based Child Health, Centre for
Paediatric Epidemiology and Biostatistics, Institute of Child Health, 30,
Guilford Street, London WC1N 1EH, UK. E-mail: firstname.lastname@example.org
the 20th century. At the start of the 20th century, such deaths
were attributed to overlying, particularly by drunken mothers.1
By the 1940s, as more deaths were investigated by autopsy,
pathologists realized that few deaths were due to maternal
overlying, and alternative mechanisms for ‘accidental
mechanical suffocation’ were sought. In 1944, Abramson, a
pathologist in New York State, noted that two-thirds of infants
dying from mechanical suffocation were found face down,
contrary to the usual sleeping position for infants at the time.2
His observations, which were corroborated by reports in the UK
Sudden unexpected unexplained infant death, now known as
sudden infant death syndrome (SIDS), was recognized as a
major cause of infant death in the UK and USA throughout
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SLEEPING POSITION AND SIDS
and Australia3,4led to a health promotion campaign that
recommended avoidance of the front position.5
The campaign was short-lived. In 1945, a paediatrician,
Woolley, rejected Abramson’s hypothesis of suffocation on the
front based on experiments in which he had covered babies’
faces with layers of blankets.6He reported that the oxygen
content of the air breathed by the babies only fell when they
were covered with a rubber sheet and that babies moved if
breathing was obstructed. He also criticized the explanation of
suffocation because it ‘instilled guilt and self-incrimination in
Emergence of alternative explanations for death, such as
unrecognized infection4,7,8inhalation of vomit9and hyper-
sensitivity reaction to inhaled milk,10further strengthened the
argument against the suffocation hypothesis and highlighted
the need for data on risk factors. The first published
case–control study was started in 1956 in the USA,11and in
1958, a similar study in the UK was the first to measure infant
sleeping position in SIDS victims and live control babies.12At
around the same time, it became increasingly common to
advocate sleeping on the front. We now know that front
sleeping is a major cause of SIDS. We wanted to know whether
systematic review of the evidence could have reversed this
harmful advice sooner or whether variation in the association
between sleeping on the front and SIDS was consistent with
recommendations at the time. We did a systematic review and
meta-analysis of the effect of front and side sleeping on the risk
of SIDS, and an historical review of recommendations on infant
sleeping position in books and pamphlets on infant care
available in the UK between 1940 and 2002. We focussed on
how the strength of the evidence for a harmful effect of front
sleeping changed before and after advice changed in favour of
avoidance of the front position. We hypothesized that the effect
of the front position on SIDS might differ depending on
whether health advice favoured front or not as families that
adopt health advice are likely to be at lower risk of SIDS.
We reviewed recommendations on infant sleeping position in
books or pamphlets available in the UK from 1940 to 2002.
We chose 1940 to include a period before the front position
was widely advocated. We searched the Modern Medicine
Collection at the Wellcome Trust library, and, because of a lack
of more recent texts, the British Medical Association library
from 1965 to 2002. We included any book or pamphlet that
referred to the care of normal term infants aged ?6 months,
and mentioned infant sleeping position. Searches used the
library indexing system for books on infant care and we also
searched electronically using terms for paediatric, parent, and
baby (details of search strategy available from authors).
One reviewer (S.S. or M.H.), assessed whether texts met the
inclusion criteria and prepared a hard copy file with the extract
and book title but not the date of publication. A second
reviewer (R.G.) categorized the recommendation as favouring
front, back, side, or non-front position(s), or neutral if all or
none were implicitly or explicitly favoured. A second reviewer
(S.S.), independently categorized one-third of the texts and
there was complete agreement with the first reviewer.
We included any case–control or cohort study that compared
the risk of SIDS in infants sleeping on their front, side, or back.
Studies had to be based on SIDS infants and live healthy control
infants from the same community. We searched for any
comparative study of infant sleeping position and SIDS in
MEDLINE (1966–2002) and EMBASE (1980–2002), using a
detailed search strategy (available from the authors), and
reference lists of review articles, a PhD thesis on the history of
SIDS,13and included studies. Abstracts were scanned by one
reviewer (S.S., M.H., or R.G.), and full texts of potentially
eligible studies retrieved. R.G. and S.P. jointly extracted data
from included studies.
We used data on the position in which the infant was placed to
sleep before death or interview, or if lacking, data on usual
position, or position found. If usual position was measured at
multiple ages, we used results closest to 3 months of age. We
recorded the method of selection of cases and controls,
matching criteria, if any, and whether data collection methods
differed in cases and controls.
Our primary aim was to compare the risk of SIDS in infants
sleeping front and back. As some studies did not separately
report side and back positions, we also compared front with
non-front positions. However, grouping side with back will
attenuate the observed risk associated with the front position if
the side position is also harmful. We therefore calculated odds
ratios (ORs) for SIDS associated with sleeping front vs back,
front vs non-front, and side vs back.
To avoid confounding, we used the unadjusted matched OR if
reported. Otherwise we calculated the unmatched OR.14Because
studies differed in their design, populations, and methods, we
used a random effects model in which it is assumed that the
observed ORs are sampled from a common distribution around
a mean effect with variance measured by the heterogeneity
parameter. We estimated 95% confidence intervals (CIs) and
considered a P-value ?0.05 as statistically significant. Hetero-
geneity in the OR for SIDS was assessed by the chi-squared test
(Q-test) and quantified using I2which reflects the proportion of
variation that is not due to sampling error.15The possibility of
publication bias was evaluated using funnel plots and the Egger
and Begg tests.16,17
We determined the year at which there was a statistically
significant association between front or side sleeping positions
and SIDS by using a cumulative meta-analysis based on year of
publication as described by Lau.18The overall heterogeneity
was used in the calculation of the CIs for the cumulative OR at
every step using a random effects model. We applied recursive
cumulative meta-analysis to examine the direction and
magnitude of the relative changes in the cumulative evidence
as a function of the cumulative sample size.19,20At the end
of every information period j, the ratio (cumulative ORj)/
(cumulative ORj ? 1) was assessed and compared with unity. If
larger than one, this was interpreted as a ‘move’ of the evidence
towards defining the front position as more harmful than in the
previous information period.
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Appendix: Hierarchical Bayesian model
for case-control studies (front vs non-front)
Consider the data in the following 2 ? 2 table
The number of children sleeping front in cases and controls
follow a binomial distribution (B) with probabilities
Then we parameterize as
then, logOR is regressed as
where now the intercept a is the adjusted log OR, D is the mean
date of recruitment, and ?1is the coefficient for the dependence
of the OR on the prevalence of front sleeping.
logOR ? a ? ?1u ? ?2D
logit(P(front?cases)) ? u?logOR
logit(P(front?control)) ? u
Fcases ˜ B(P(front?case), ncases)
Fcont ˜ B(P(front?control), ncont)
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