We identified four eligible studies from five publications conducted in developed countries and published since 1997 that evaluated the relationship of breastfeeding and SIDS.116-120 Three
were case-control studies and the fourth, a case-cohort study (a cohort study analyzed as a case- control study). The methodological quality ranged from grades B to C. There were a total of 769 cases of SIDS and 2,681 controls.
All studies were designed to evaluate a broad range of potential risk factors for SIDS. The studies differed in their description of the duration of breastfeeding. Similarly, the studies varied in their definition of the time interval when SIDS occurs in infants, but all studies reported autopsy-confirmed diagnoses. The mean age of the infants with SIDS ranged from 2 to 19 weeks. All studies provided adjusted odds ratios for the association of breastfeeding and SIDS. Three of the four studies identified statistically significant increased risk of SIDS in bottle-fed infants.117,119,120 Two studies reported that the risk of SIDS was twice or more for non-breastfed
infants compared with some or ever breastfed infants.117,119 One study reported an approximately two times increased risk of SIDS among those breastfed less than 2 weeks compared with those breastfed more than 2 weeks.120 One case-cohort study (a cohort study analyzed as case-control study) did not find a statistically significant increased risk of SIDS in bottle fed infants.118
Meta-analysis results. Because of the limitations of the previous meta-analysis, we elected to conduct our own meta-analysis using only studies that provided an objective definition of SIDS (autopsy confirmed SIDS among infants 1 week to 1 year of age), clear reporting of breastfeeding data, and outcomes adjusted for important confounders or risk factors (e.g., sleeping positions, maternal smoking, and socioeconomic status). Four studies included in the previously published meta-analysis121-124 and two studies published since 1997 met the eligibility criteria.118,120 The majority of the studies provided data on ever versus never breastfeeding and this was combined using a random effects model. The results from our meta-analysis found that ever breastfeeding was associated with a reduction in both crude and adjusted risk of SIDS (crude OR 0.41; 95%CI (0.28, 0.58), and adjusted OR 0.64; 95%CI (0.51, 0.81), respectively); both estimates were statistically significant with a reduction in SIDS for the ever breastfed infants.
Figure 10. Random effects model of summary estimate evaluating the association of breastfeeding and SIDS
Conclusion
Results from the previously published meta-analysis of case-control studies concluded that an overall crude risk of SIDS was twice as great for formula-fed infants compared with breastfed infants. The conclusion may be biased because the reported association was not adjusted for potential confounders. Misclassification biases may occur because of differences among studies with regard to definitions of breastfeeding exposure, definitions of SIDS, and the wide age range of population included in the studies.
Findings from the four studies published subsequent to the meta-analysis in developed countries concurred with the findings from the meta-analysis. All studies reported autopsy- confirmed diagnoses of SIDS and adjusted for potential confounders. However, the definitions of breastfeeding exposure and the time intervals accepted for defining SIDS varied across studies. Three of four studies reported statistically significant increased risk of SIDS associated with non- breastfeeding or reduced duration of breastfeeding and the fourth study reported a statistically non-significant increased risk.
Our meta-analysis included only studies that reported clear definitions of exposure, outcomes, and results adjusted for well-known confounders or risk factors for SIDS. The
summary estimate found a statistically significant adjusted odds ratio for an association between breastfeeding and a reduced risk of SIDS (adjusted OR 0.64, 95%CI 0.51 - 0.81). We conclude that there is a relationship between breastfeeding and a reduced risk of SIDS.
R e d u c tio n in ris k w ith b re a s tfe e d in g A d ju s te d O R o f S ID S In c re a s e in ris k w ith b re a s tfe e d in g
.1 .5 .8 1 1 .5 2 5 S tu d y A d ju s te d O R o f S ID S (9 5 % C I) % W e ig h t Q u a lity B ro o k e 1 9 9 7 0 .9 9 ( 0 .3 7 , 2 .5 6 ) 5 .1 A W e n n e rg re n 1 9 9 7 0 .5 0 ( 0 .4 5 , 0 .7 7 ) 2 8 .6 A F le m in g 1 9 9 6 1 .0 6 ( 0 .5 7 , 1 .9 8 ) 1 0 .6 A M itc h e ll 1 9 9 3 0 .5 3 ( 0 .3 8 , 0 .7 4 ) 2 3 .6 B V e n n e m a n n 2 0 0 5 0 .5 9 ( 0 .3 6 , 0 .9 4 ) 1 5 .5 B M itc h e ll 1 9 9 7 0 .7 6 ( 0 .4 0 , 1 .4 7 ) 9 .9 B M itc h e ll2 1 9 9 7 1 .0 7 ( 0 .4 7 , 2 .4 3 ) 6 .7 B O v e ra ll 0 .6 4 ( 0 .5 1 , 0 .8 1 ) 1 0 0 .0
96
Table 20. Summary of systematic review/meta-analysis on the relationship between breastfeeding and Sudden Infant Death Syndrome (SIDS) Author
year description Study Number of subjects Population /Comparator Intervention Results of SR/MA Quality
McVea 2000 MA of 23 observational studies Cases 4,251 Controls 58,055 Children who were diagnosed of SIDS Breastfeeding (any)
The pooled OR for the 23 studies using random effects model resulted in an OR = 2.11 (95% CI 1.66-2.68) i.e., the overall risk of SIDS was twice as great for bottle-fed infants compared to breastfed infants. The pooled OR from the higher quality studies also demonstrated a two
fold increase in risk among bottle fed infants OR = 2.24 The pooled OR from studies after 1988 OR = 2.32
Confounders: Individual studies adjusted for potential confounders; 6 studies reported adjusted OR; 4 studies reported no protective effect of breastfeeding, while 2 reported adjusted OR that remained significant.
Dose-response relationship: 4 out of 9 studies showed a dose response trend with the risk of SIDS increasing with increasing formula feeding. None of the studies had sufficient power to demonstrate a statistically significant difference between partial vs. no breastfeeding.
C
97
Table 21. Summary of case-control studies on the relationship between breastfeeding and sudden infant death syndrome (SIDS) included in the meta- analysis OR* (95% CI) Author year Country Cases (N) Control (N) Definition of SIDS Mean Age at Dx (week) Breastfeeding group Comparator
group Crude Adjust Potential confounders adjusted
Quality and limitations Wennegran, 1997 Alm 2002 Scandinavia
244 869 Validated definition 16 Ever (At the time of
death) Never 0.59 (0.4, 0.83) 0.5 (0.45, 0.77)
Sleep position, maternal smoking, bottle feeding at the time of death and age at the time of death
A Fleming,
1996 UK
195 780 7d-1y nd Ever Never 0.5 (0.35,
0.71)
1.06 (0.57, 1.98)
Maternal age, gestation, birth weight, exposure to tobacco, sleeping environment A Brooke, 1992 UK (Scotland)
147 275 7d-1y 15-18 Ever (At the time of death) Never (0.11, 0.22 0.47) 0.99 (0.37, 2.56)
Exposure to Parental smoking, sleep position, old mattress use, maternal age <27, deprivation score of 7, drug treatment in previous wk, marital status of the mother, SE status, gender of the infant, birth weight etc
A Mitchell, 1993 New Zealand Population 1987-1990 460 1757 >28d-1yr nd Ever
(At the time of discharge from obstetric dept)
Never 0.42(0.33, 0.53) 0.53(0.38, 0.74)
Region, time of day, baby’s age, antenatal class, school leaving age of mother, marital status of mother, sex of baby, admission to neonatal unit, number of previous pregnancies, socioeconomic status, birthweight, gestational age, race of baby, season, mothers age at first pregnancy, mothers age at birth, sleeping position, bed sharing with another person, maternal smoking and breastfeeding
B Vennemann 2005 Germany 333 998 8d-1 yr 19 >2 wk <2 wk 0.19 (0.14, 0.25) 0.59 (0.36, 0.94)
Maternal age, family status, smoking in pregnancy. Previous live births and
socioeconomic status B
79 679 Any at initial contact None at initial contact 0.60 (0.35, 1.03) 0.76 (0.40,1.47) Mitchell 1997a New Zealand 38 588 29d-1 yr 2.6-9 Any at 2 mo None at 2 mo (0.41, 0.76 1.39) 1.07 (0.47, 2.43)
Maternal age, marital status, age mother left school, previous number of
pregnancies, infant’s sex, ethnicity of infant, birthweight, sleep position, and bed sharing/maternal smoking combinations
B
a