To what extent does confounding explain the association between breastfeeding duration and cognitive development up to age 14? Findings from the UK Millennium Cohort Study

Abstract

Background: Breastfeeding duration is associated with improved cognitive development in children, but it is unclear whether this is a causal relationship or due to confounding. This study evaluates whether the observed association is explained by socioeconomic position (SEP) and maternal cognitive ability.

Methods: Data from 7,855 singletons born in 2000-2002 and followed up to age 14 years within 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 breastfeeding duration groups using multivariable linear mixed-effects models (repeated outcome measures).

Results: At all ages, longer breastfeeding durations were associated with higher cognitive scores after accounting for the child's own characteristics. Adjustment for SEP approximately halved the effect sizes. Further adjustment for maternal cognitive scores removed the remaining associations at age 5, but not at ages 7, 11 and 14 (e.g.: verbal scores, age 14; breastfed ≥12 months vs never breastfed: 0.26 SD; 95%CI: 0.18, 0.34).

Conclusion: The associations between breastfeeding duration and cognitive scores persist after adjusting for SEP and maternal cognitive ability, however the effect was modest.

Fig 1. Association between breastfeeding duration and cognitive development among children from the UK Millennium Cohort Study.

Fig 2. Directed acyclic graph used for the evaluation of the association between breastfeeding duration and cognitive development.

Fig 3. Flowchart of study participants included in the analysis, UK Millennium Cohort Study.

*Those not present at age 14 were excluded because data for maternal cognitive scores were assessed during that evaluation. **Data missing for the analysis of verbal scores.

Fig 4. Association between breastfeeding duration (any breastfeeding) and standardised cognitive verbal scores (mean: 0; SD: 1)between ages 5 and 14, UK Millennium Cohort Study (n = 7,855).

All categories of BF duration are compared to “Never breastfed” as the reference category. Model 1: Adjusted for gestational age at birth, maternal ethnicity and languages spoken in household. Model 2: Adjusted for Model 1 + Socioeconomic position (maternal education and highest social class in household). Model 3: Adjusted for Model 2 + other confounding factors (older siblings in household, maternal age, mother working outside the home, partnership status, and maternal smoking during pregnancy). Model 4: Adjusted for Model 4 + Maternal cognitive score.

Fig 5. Association between breastfeeding duration (any breastfeeding) and standardised cognitive spatial scores (mean: 0; SD: 1) between ages 5 and 11, UK Millennium Cohort Study (n = 7,582).

All categories of BF duration are compared to “Never breastfed” as the reference category. Model 1: Adjusted for gestational age at birth, maternal ethnicity and languages spoken in household. Model 2: Adjusted for Model 1 + Socioeconomic position (maternal education and highest social class in household). Model 3: Adjusted for Model 2 + other confounding factors (older siblings in household, mother working outside the home, partnership status, maternal alcohol use during pregnancy and smoking during pregnancy). Model 4: Adjusted for Model 3 + Maternal cognitive score.

Fig 6. Comparison of the coefficients for breastfeeding duration, markers of socioeconomic position and maternal cognitive scores on verbal and spatial cognitive outcomes* at ages 14 and 11, respectively.

The models adjusted for all potential confounders of the association between breastfeeding duration and cognitive scores, and include an interaction between breastfeeding duration and age of outcome measurement. *Standardised maternal cognitive scores (Mean: 0; SD: 1).