Background Duration of breastfeeding is associated with improved cognitive development, but it is unclear whether this relationship is real or due to confounding. This study evaluates how much of the observed association is explained by confounding of socioeconomic position (SEP) and maternal cognitive ability.
Methods Data for 6,421 singleton children, born in 2000–2002 and followed up to 14 years of age as part of the UK Millennium Cohort Study, were analysed. Mothers reported breastfeeding duration, and children’s cognitive abilities were assessed at 5, 7, 11, and 14 years using validated measures. Standardised verbal (age 5 to 14) and spatial (age 5 to 11) cognitive scores were compared across groups of breastfeeding using multivariable linear regression models, adjusting for SEP, maternal cognitive ability, and other confounders/mediators.
Results At age 5, longer breastfeeding duration showed a graded association with higher verbal cognitive scores (coefficient, ≥12 months vs never breastfed: 0.34; 95%CI: 0.25 to 0.44). Adjustment for SEP approximately halved the effect sizes and further adjustment for maternal cognitive scores removed the remaining association (coefficient: 0.06; 95%CI: -0.03 to 0.14). Findings were similar for ages 7 and 11 but not for age 14, in which the score of those who breastfed for ≥12 months remained 0.20 s.d. (95%CI: 0.08 to 0.31) higher than the score of those never breastfed, after full adjustment. The crude results for spatial scores at age 5 showed that participants breastfed for ≥12 months scored 0.21 s.d. (95%CI: 0.12 to 0.31) higher than those never breastfed. After full adjustment, the differences vanished (coefficient: -0.03; 95%CI: -0.12 to 0.07). However, those participants breastfed for ≥4 and <6 months scored 0.10 s.d. (95%CI: 0.02 to 0.18) higher than those never breastfed, after full adjustment. Results were similar for ages 7 and 11. Exclusive breastfeeding showed similar patterns. However, even after full adjustment, a duration of ≥4 months was associated with improved verbal scores at age 14 (coefficient: 0.11; 95%CI: 0.02 to 0.20) and spatial scores at age 7 (coefficient: 0.09; 95%CI: 0.01 to 0.17) and 11 (coefficient: 0.09; 95%CI: 0.01 to 0.18).
Conclusion The positive associations between any breastfeeding duration and cognitive development were explained in full after adjusting for SEP and maternal cognitive scores, except at age 14 (verbal). Exclusive breastfeeding duration seemed to be associated with improved cognitive verbal scores at age 14 and spatial scores at ages 7 and 11 after full adjustment, although with modest effect sizes.
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