The effects of breastfeeding exclusivity on early childhood outcomes

Abstract

Objectives: We examined the relationship between breastfeeding exclusivity and duration and children's health and cognitive outcomes at ages 2 and 4 years.

Methods: We used the Early Childhood Longitudinal Study-Birth Cohort, a nationally representative sample of 10,700 children born in the United States in 2001. Parent interviews and child assessments were conducted in measurement waves at 9 months, 2 years, 4 years, and in kindergarten, with the focus on ages 2 and 4 years. We employed propensity scores as a means of adjusting for confounding involving observed characteristics.

Results: Outcome analyses using propensity scores showed some small effects of breastfeeding on key outcomes at age 4 years but not at age 2 years. Effects appeared to be concentrated in reading and cognitive outcomes. Overall, we found no consistent evidence for dosage effects of breastfeeding exclusivity. Our sensitivity analyses revealed that a small amount of unobserved confounding could be responsible for the resulting benefits.

Conclusions: Our study revealed little or no effect of breastfeeding exclusivity and duration on key child outcomes.

FIGURE 1—

Comparative line plots of the significant estimated effects of breastfeeding exclusivity (BFE) exposures on child outcomes at age 2 and 4 years: Early Childhood Longitudinal Study—Birth cohort, United States, 2001. Note. BMI = body mass index. Each line represents the results from 1 outcome model; each point on the line is the estimated coefficient of a BFE exposure for that outcome. The reference category is children who were never breastfed. The x-axis delineates the duration of the BFE exposure, and the y-axis describes the magnitude of the effect size. Patterns of BFE dosage in the figure can be found by following the line for an outcome from left to right as BFE duration increases. This display only includes models with significant results.

FIGURE 2—

Illustration of the γ sensitivity factor calculation procedure and meaning of the γ statistic: Early Childhood Longitudinal Study—Birth cohort, United States, 2001. Note. Building on Rosenbaum, we calculated γ statistics to assess the extent to which unobserved confounding could affect our estimates. The calculation proceeded in the following steps: (1) estimate potentially confounded effect using inverse probability of treatment weights (IPTWs) at determined level of significance based on critical value. Convert estimate to standardized coefficient (B); (2) multiply the standard error of the estimate by the critical value to determine the "threshold" value of standardized breastfeeding exclusively (BFE) coefficient needed for the effect to reach significance; and (3) calculate the difference between the potentially confounded estimated effect and the threshold value for a significant effect to determine the amount (in terms of effect size) by which the point estimate may be inflated or suppressed by unobserved confounding.