Context: Sudden infant death syndrome (SIDS) is a leading cause of postneonatal infant mortality. Our previous meta-analyses showed that any breastfeeding is protective against SIDS with exclusive breastfeeding conferring a stronger effect.The duration of breastfeeding required to confer a protective effect is unknown.

Objective: To assess the associations between breastfeeding duration and SIDS.

Data sources: Individual-level data from 8 case-control studies.

Study selection: Case-control SIDS studies with breastfeeding data.

Data extraction: Breastfeeding variables, demographic factors, and other potential confounders were identified. Individual-study and pooled analyses were performed.

Results: A total of 2267 SIDS cases and 6837 control infants were included. In multivariable pooled analysis, breastfeeding for <2 months was not protective (adjusted odds ratio [aOR]: 0.91, 95% confidence interval [CI]: 0.68-1.22). Any breastfeeding ≥2 months was protective, with greater protection seen with increased duration (2-4 months: aOR: 0.60, 95% CI: 0.44-0.82; 4-6 months: aOR: 0.40, 95% CI: 0.26-0.63; and >6 months: aOR: 0.36, 95% CI: 0.22-0.61). Although exclusive breastfeeding for <2 months was not protective (aOR: 0.82, 95% CI: 0.59-1.14), longer periods were protective (2-4 months: aOR: 0.61, 95% CI: 0.42-0.87; 4-6 months: aOR: 0.46, 95% CI: 0.29-0.74).

Limitations: The variables collected in each study varied slightly, limiting our ability to include all studies in the analysis and control for all confounders.

Conclusions: Breastfeeding duration of at least 2 months was associated with half the risk of SIDS. Breastfeeding does not need to be exclusive to confer this protection.