Breastfeeding Duration and IQ: The Evidence
Breastfeeding for less than 2 years does not meaningfully decrease IQ, though breastfeeding for any duration beyond 1 month appears to confer a modest cognitive advantage of approximately 3-5 IQ points compared to minimal or no breastfeeding. The critical period appears to be the first 6-12 months, with limited evidence supporting additional cognitive benefits from extending breastfeeding to 2 years specifically for IQ outcomes 1.
Key Evidence on Breastfeeding Duration and Cognitive Outcomes
The Dose-Response Relationship
The relationship between breastfeeding duration and IQ is not linear beyond the first year:
Meta-analyses demonstrate a pooled estimate of 3-5 IQ points favoring children who have been breastfed compared to those who were not, based on observational studies 1.
The largest randomized controlled trial (PROBIT study in Belarus, N=13,889) showed that breastfeeding promotion led to higher IQ scores at age 6.5 years, with a mean difference of +7.5 points for verbal IQ and +5.9 points for full-scale IQ 1, 2.
A prospective Brazilian cohort study found that breastfeeding for 12 months or more resulted in 3.76 higher IQ points at age 30 compared to breastfeeding less than 1 month, with dose-response effects observed 3.
Critical Threshold: The First Month Matters Most
Recent high-quality evidence suggests the primary cognitive benefit occurs early:
A Danish prospective cohort (N=1,385) found that breastfeeding duration of 1 month or shorter was associated with approximately 3 points lower IQ compared to longer periods, but there was no dose-response relationship beyond 1 month 4.
The main cognitive difference exists between breastfeeding ≤1 month versus >1 month, with no clear additional benefit from extending beyond this threshold in terms of IQ points 4.
The Confounding Problem
The association between breastfeeding and IQ is heavily confounded by socioeconomic and maternal factors:
A Dutch population-based cohort (N=3,761) found that the initial association of 0.32 IQ points per additional month of breastfeeding was reduced to 0.09 points (non-significant) after adjusting for maternal IQ, sociodemographic factors, and parental lifestyle 5.
A UK twin study (N=11,582) found that breastfeeding was associated with only a small advantage in IQ at age 2 in girls but not boys, and was not associated with IQ growth trajectories through adolescence 6.
Socioeconomic status, maternal education, and maternal IQ are powerful confounders that explain much of the observed association between breastfeeding duration and child cognitive outcomes 1, 5.
The Two-Year Recommendation Context
WHO Guidelines and Their Rationale
The WHO recommends exclusive breastfeeding for the first 6 months, followed by breastfeeding supplemented with complementary foods until age 2 years or beyond 1.
This recommendation is based on multiple health outcomes including reduced infection rates, lower childhood obesity risk, and improved maternal health—not solely on IQ outcomes 1.
The most active period of brain development occurs in the first 1000 days of life (conception through age 2 years), during which nutrition provides fuel for brain growth and neural pathway refinement 1.
Evidence Limitations for the 2-Year Threshold
The scientific evidence does not unequivocally demonstrate that human milk leads to better cognitive development, and studies with experimental designs are scarce 1.
Not all studies have found positive associations between breastfeeding parameters and child cognitive performance 1.
The mixture of feeding methods (direct breastfeeding, pumped milk, donor milk, formula, and combinations) complicates outcome comparisons 1.
Clinical Bottom Line
For IQ outcomes specifically, the evidence supports breastfeeding for at least 1 month, with optimal cognitive benefits likely achieved by 6-12 months of any breastfeeding. Extending breastfeeding to 2 years provides numerous other health benefits for mother and child but does not appear to confer substantial additional IQ advantages beyond the first year 1, 3, 4.
Important Caveats
The cognitive benefits of breastfeeding are modest (3-5 IQ points) and may be largely explained by confounding factors including maternal intelligence, socioeconomic status, and parenting behaviors 1, 5.
Breastfeeding duration recommendations should be based on the totality of health outcomes—including reduced infections, lower obesity risk, and maternal health benefits—not IQ alone 1.
Individual circumstances, maternal choice, and practical feasibility must be considered, as the pressure to breastfeed for extended periods based solely on cognitive outcomes is not strongly supported by current evidence 1.