摘要：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. In terms of its public health profile, breastfeeding has few competitors. The American Academy of Pediatrics (AAP) declares that Pediatricians and other health care professionals should recommend human milk for all infants in whom breastfeeding is not specifically contraindicated and provide parents with complete, current information on the benefits and techniques of breastfeeding. 1 (p498) By contrast, the scientific evidence on the benefits of breastfeeding offers mixed and weak support for the relationship between breastfeeding and key child outcomes. 2 Researchers recognize the methodological challenges of breastfeeding research, such as confounding by measured and unmeasured variables. 3 Mothers self-select breastfeeding, and the characteristics that guide this choice likely affect child outcomes. Variability in attending to potential confounding may explain why the putative effects of breastfeeding vary across studies. Another issue in uncovering the effects of breastfeeding is defining the exposure; breastfeeding can be measured by duration, exclusivity, or volume of breast milk consumed. Breastfeeding is believed most effective when mothers do so exclusively. 3,4 Reflecting this belief, the Healthy People 2010 goals include a target for exclusive breastfeeding at 6 months of 25%. 5 Likewise, the AAP recommends that solid foods not be introduced before a child’s 4-month birthday. 1,6 Therefore, one explanation for the mixed effects of breastfeeding is that women who breastfeed vary in how they breastfeed, and children who are exclusively breastfed may benefit more than those who are nonexclusively breastfed. In addition, further differentiating breastfeeding beyond simply breastfed or not complicates causal inference. Whether a woman breastfeeds a child at all is a matter of one behavior: breastfeeding initiation. Exclusivity, however, involves other processes, such as discontinuing breastfeeding and initiating solid foods. In general, the more processes shaping an exposure, the more possibilities for self-selection and unobserved confounding. 7 We considered the previous research available on the effect of breastfeeding “dose” (exclusivity and duration) on child outcomes and then examined the issue using a large nationally representative panel data set. Relative to past research, our analyses adjusted for a rich set of covariates and employed improved methodology; notably, propensity scores (PSs) and analyses that considered the sensitivity of key findings to unobserved confounding.
