Analysis: Breastfeeding Rates
National Immunization Survey (NIS)
Analysis of Breastfeeding Practice Data by Year of Child Birth
We combine survey years to calculate breastfeeding indicators by year of child birth (Smith et al., 2006). Because children were 19–35 months of age at the time of the parent interview through NIS survey year 2010 and 19-35 months of age any time during the survey year quarter starting with the 2011 NIS survey, each survey year represents children born over three years (see Table A2).
For each birth year, we estimate the percentage of infants in the following categories: “ever breastfed,” “breastfed at 6 months,” “breastfed at 12 months,” “exclusively breastfed through 3 months,” and “exclusively breastfed through 6 months.” whose mothers started breastfeeding.
- “Ever breastfed” is estimated by the question: “Was [child] ever breastfed or fed breast milk?”
- “Breastfeeding duration” is estimated by the question: “How old was [child] when he/she completely stopped breastfeeding or being fed breast milk?”
- Because exclusive breastfeeding is defined as ONLY breast milk (no solids, water, or other liquids); the duration of exclusive breastfeeding is estimated by the two survey questions about age, including the age of the child when he/she was first fed formula, and the age of the child when he/she was first fed anything other than breast milk or formula (including water).
We also calculate the percentage of breastfed infants who are supplemented with infant formula before they are 2 days, 3 months, and 6 months old. The rates of formula supplementation (with or without other supplementary liquids or solids) before 2 days, 3 months, and 6 months are calculated among infants who are breastfeeding at each respective age, whereas the breastfeeding initiation, duration, and exclusivity rates are calculated among all infants surveyed.
Breastfeeding rates among children in a birth year are released when approximately two thirds of the children born in that year have been surveyed. In the past, the rates were labeled provisional until they were replaced the following year with final rates based on all children surveyed in the birth year. Starting from 2010 births for landline rates and 2009 births for dual frame rates, CDC now reports one final rate based on 2 survey years only. This is because adding a third year of survey data to a birth cohort has little impact on the breastfeeding rate and results in a margin of error that is only about 20% smaller than with two survey years. All analyses are conducted using statistical software that accounts for complex sample design.
|Table A2. Years of breastfeeding data from National Immunization Survey used to calculate breastfeeding rates for each birth cohort1|
|NIS survey year||Birth mon/yr represented||Birth Year 2009||Birth Year 2010||Birth Year 2011||Birth Year 2012||Birth Year 2013||Birth Year 2014||Birth Year 2015|
1For the 2009 birth cohort, the LAST two survey years of data were used to estimate dual-frame breastfeeding rates because the cellular telephone sample was not added until 2011. Starting with the 2010 birth cohort, CDC includes only the FIRST two survey years of data for annual dual-frame estimates of breastfeeding rates.