Breastfeeding increases an interactive process between mother and infant (Wood & Sanders, 2017) and is an optimal feeding method for infants. Benefits of breastfeeding affect infant health, a child's long-term health, and maternal health. The World Health Organization (WHO, 2008) and the American Academy of Pediatrics (2012) recommend exclusive breastfeeding either through direct breastfeeding or expressed milk in a bottle for the first 6 months and partial breastfeeding with combination of complementary foods for at least 1 year and preferably for up to 2 years or beyond. Therefore, breastfeeding initiation, duration, and exclusivity are major outcomes for breastfeeding intervention studies.
Randomized controlled trials (RCTs) represent the gold standard for evaluating the effectiveness of interventions when they are appropriately designed, tested, and analyzed. They provide causal relationships between independent variables and dependent variables. The RCT method minimizes potential threats to internal validity, strengthening validity of the inference and evidence (Polit & Beck, 2017). Earlier, we proposed that an intervention should not directly try to change problems such as breastfeeding initiation, duration, and exclusivity, but instead focus on a modifiable cause of breastfeeding discontinuation (e.g., maternal perception of infant behavior: the modifiable cause of perceived insufficient milk) (Wood, Woods, Blackburn, & Sanders, 2016). Choice of outcomes should be attributable to the target of the intervention: the modifiable cause of breastfeeding discontinuation. Primary outcomes are the most important outcomes with the greatest intervention effect. Secondary or tertiary outcomes have intermediate connection to the intervention and the least intervention effect.
The purpose of this review is to analyze measures for breastfeeding that were used to examine outcomes including at a minimum, breastfeeding initiation, duration, and/or exclusivity in RCTs. Strengths as well as limitations of the outcome measures are analyzed. Recommendations are generated for improvement of the outcome measures.
A literature search was conducted with consultation from a nursing liaison librarian at the University of Washington, using the PRISMA guidelines (Moher, Liberati, Tetzlaff, Altman, & The PRISMA Group, 2009). Search engines: PubMed/MEDLINE (National Library of Medicine), CINAHL Plus (EBSCO), and PsycINFO (EBSCO) were used. Search terms included breastfeeding, feeding behavior, prenatal/patient education, health promotion, social support, perinatal/prenatal/intrapartum/postpartum care, and postpartum period. Additional limits included RCTs, humans, English language, female, and January 2006 to March 2017. Through this process, 229 articles were retrieved as follows: PubMed/MEDLINE (n = 168), CINAHL Plus (n = 42), and PsycINFO (n = 19). After two duplicates were eliminated, 227 abstracts were reviewed. Abstract selection criteria included breastfeeding intervention tested, control group used, randomization to study group, and breastfeeding status as study outcomes; 208 articles were excluded. Then, 19 full-text English articles were reviewed after the study criteria were met. Criteria were studies of outcomes of breastfeeding initiation, duration or exclusivity as a primary, secondary, or tertiary outcome, healthy term singleton infants and healthy mothers, no smoking, alcohol or drug abuse during pregnancy and throughout the postpartum period. Exclusion criteria included studies targeting adolescents, mothers with HIV-positive status, and study outcomes focused on breastfeeding-friendly hospital practices. Ten articles were excluded due to incongruity with exclusion criteria. Nine articles met the selection criteria and were included in the review (Figure 1).
Standardized Definitions of Breastfeeding
The Interagency Group for Action on Breastfeeding (IGAB) (Labbok & Krasovec, 1990) first developed standardized terminology to assess breastfeeding and test interventions. Definitions include full breastfeeding, partial breastfeeding, and token breastfeeding. The terminology was categorized this way because of a dose response relationship between the different levels of breast milk consumed and health outcomes, including morbidity and mortality in the infant and fertility consequences in the mother.
In 1991 WHO, for simplicity, proposed modifications to the definitions as outlined by IGAB. The definition then focused solely on what enters the infant's mouth. There are three types of breastfeeding: exclusive breastfeeding, predominant breastfeeding, and complementary breastfeeding, and two methods of infant feeding: breastfeeding and bottle feeding. In 2008, WHO changed exclusive breastfeeding to infants exclusively getting breast milk in addition to allowing oral rehydration salt drops, and syrups that contain vitamins, minerals, and medicines.
There are similarities and differences in breastfeeding definitions between IGAB (Labbok & Krasovec, 1990) and WHO (1991 , 2008). Both agencies do not differentiate between the physical and psychological effects of direct breastfeeding on the breast and those of indirect breastfeeding such as bottle feeding with expressed milk. The breastfeeding category of “token” that includes infant comfort and consoling measures was omitted by WHO. The terminology of WHO has changed from “almost exclusive breastfeeding” to “predominant breastfeeding.” There is an acceptance of oral rehydration salt in the category of exclusive breastfeeding in the WHO (2008) terminology. For the methods of infant feeding in the WHO (2008) definition, it is not clear that which category expressed milk in a bottle falls.
The statement of age-based feeding recommendations by WHO (1991) is consistent with the health benefits and protective effects in the systematic review by Kramer and Kakuma (2012). Exclusively breastfeeding mother and infant dyads for 6 months received several more health benefits than mother and infant dyads who were exclusively breastfed for 3 to 4 months followed by a mixed feeding. Mixed feeding is defined as in a combination of breast milk and formula milk. Those health benefits for exclusive breastfeeding for the first 6 months include lower risk of gastrointestinal infection for the infant, more rapid maternal weight return to prepregnancy size after childbirth, and delayed onset of menstrual periods (Kramer & Kakuma). Early initiation of breastfeeding refers to mothers who put their infant to the breast within 1 hour after birth (WHO, 2017). Breastfeeding duration was equivalent with the infant age when the mother completely stopped breastfeeding.
Operational Definitions of Breastfeeding
The types of breastfeeding were defined using different terminology across different studies. Five out of nine studies adapted the definitions of “exclusive breastfeeding” from WHO (1991), two studies from WHO (2008), and in two studies definitions were adapted from IGAB (Labbok & Krasovec, 1990) (Table 1). Noel-Weiss, Rupp, Cragg, Bassett, and Woodend (2006) adapted six categories by IGAB (Labbok & Krasovec, 1990) and delineated exclusive breastfeeding into directly breastfeeding or expressed milk in a bottle. Moore and Anderson (2007) did not consider expressed milk by pumping to be breastfeeding because the primary outcome was success of first breastfeeding with emphasis on the sucking component of the infant, not the amount of breast milk consumed.
“Any breastfeeding” refers to any types of breastfeeding except exclusive breastfeeding (Fu et al., 2014 ; Kronborg, Maimburg, & Vaeth, 2012 ; Moore & Anderson, 2007 ; Su et al., 2007). “Predominant breastfeeding” contains vitamins, minerals, water, juice, or ritualistic feeds given infrequently in addition to breastfeeding (Ahmed, Roumani, Szucs, Zhang, & King, 2016 ; Mattar et al., 2007). However, Ahmed et al. (2016) recategorized this to “partial” that includes predominant breastfeeding and formula feeding because of a low response rate in that category.
Two studies defined breastfeeding initiation differently from the WHO (1991) : Mattar et al. (2007) described breastfeeding initiation within the first 2 weeks of birth, and Moore and Anderson (2007) in the first 2 hours of birth. Duration of breastfeeding was equal to the infant's age when the mother completely stopped breastfeeding.
Types of Outcome Measures
The majority of studies measured rates of breastfeeding initiation, duration, and exclusivity as a primary and/or secondary outcome. There was variability among the studies on the time points when data were collected.
Table 2 shows primary outcomes. Primary outcomes were recorded for initiating breastfeeding by means of breastfeeding success in terms of infant breastfeeding behavior as well as breastfeeding rate at birth. Rate of exclusive breastfeeding was collected at a single point from birth to 6 months. Duration of exclusive breastfeeding was measured by number of days, weeks, and months in the first 6 months.
Secondary outcomes were recorded for the rate of exclusive breastfeeding at varied points in time or along with any or full breastfeeding rate (Table 3). Other outcomes were measured including breastfeeding problems at 1 month postpartum (Moore & Anderson, 2007), level of mother's breastfeeding knowledge (Kronborg et al., 2012), breastfeeding self-efficacy (Kronborg et al., 2012 ; Noel-Weiss et al., 2006), postpartum depression (Ahmed et al., 2016), and mother's satisfaction during breastfeeding (Kronborg, Vaeth, Olsen, Iversen, & Harder, 2007).
Data Collection Methods
Data collection methods varied among studies (Table 4). Breastfeeding initiation was assessed using chart reviews, breastfeeding observations, and online surveys by research assistants. Rate of exclusive breastfeeding was collected after the interventions that include prenatal lactation consultant support (Mattar et al., 2007), postpartum support by RNs (Kronborg et al., 2007), web-based breastfeeding support (Ahmed et al., 2016 ; Giglia, Cox, Zhao, & Binns, 2015), weekly postdischarge breastfeeding telephone support by research nurses (Fu et al., 2014). Timing of data collection was different across studies, ranging from birth to 1 year. Duration of breastfeeding up to 6 months postpartum was assessed by number of weeks or days of different types of breastfeeding using either mailed or emailed questionnaire surveys, online surveys, or telephone interviews.
Definition of exclusive breastfeeding varied across studies; therefore, caution is warranted when interpreting results of collective studies. However, it is likely that exclusive breastfeeding was explained either as the infant receiving only breast milk or by inversely asking whether the infant has received anything besides breast milk. Mothers were able to answer both types of questions.
Strengths of Outcome Measures
This analysis of outcome measures reveals several strengths. Telephone interviews offered a purposeful conversation in which the interviewer asked prepared questions and mothers answered them. It was particularly useful when the definitions of breastfeeding needed clarification. Telephone interviews allow a probe into the initial responses of the mother to gain accurate information about breastfeeding. This avoids misclassifying breastfeeding categories. However, compared with face-to-face interviews, telephone interviews are more suitable for short and less complex interviews.
Self-report questionnaires is the simplest way of obtaining information concerning breastfeeding initiation, breastfeeding status, the introduction of complimentary food, and breastfeeding discontinuation. Questionnaires offer an effective way of obtaining specific information about breastfeeding from a large sample size, are cost-effective and limit interviewer bias.
Structured observations of the patterns of infant feeding behavior in terms of breastfeeding success (Moore & Anderson, 2007) provided the most accurate information about breastfeeding initiation. Feeding logs entered in the web-based monitoring system diaries (Ahmed et al., 2016) kept track of daily infant feeding including the type and method of breastfeeding that provided sufficient information about breastfeeding status through the classification of exclusive breastfeeding and partial breastfeeding at 1 month postpartum.
Limitations of Outcome Measures
The 24-hour recall may give the types of feeding at a point in time; however, this method is not a reliable representation of breastfeeding over longer recall periods (Hector, 2011 ; Noel-Weiss, Boersma, & Kujawa-Myles, 2012 ; Thulier, 2010). It is possible to resume exclusive breastfeeding on the breast after supplementing with formula feedings as an adjunct to breastfeeding during infant physiological/iatrogenic weight loss, concomitantly occurring delayed onset of lactogenesis II in the first month postpartum (Wood, Sanders, Lewis, Woods, & Blackburn, 2017). Recall bias might have resulted in misclassification and/or over- and/or underreporting of breastfeeding.
Lack of consistency in breastfeeding definitions remains a problem (Chapman & Pérez-Escamilla, 2009 ; Hector, 2011 ; Thulier, 2010). Breastfeeding definitions from IGAB (Labbok & Krasovec, 1990) and WHO (1991 , 2008) focus on the types of breastfeeding and not on the methods of breastfeeding (Noel-Weiss et al., 2012). However, separating breastfeeding from breast milk feeding is a crucial component of breastfeeding outcomes because breastfeeding involves a stronger relationship between mother and infant (Wood & Sanders, 2017), distinctive suckling patterns that contribute to health benefits and protective effects for both mothers and infants (Ip, Chung, Raman, Trikalinos, & Lau, 2009 ; Kramer & Kakuma, 2012 ; Labbok, 2015), and comfort sucking (nonnutritive sucking) that is relevant to increased alertness, social awareness, and responsiveness in infants (Nugent, Keefer, Minear, Johnson, & Blanchard, 2007). Lack of precise and consistent breastfeeding definitions lead to problems with the collection of valid and reliable information on breastfeeding outcomes, misinterpretation of data, and having difficulty with comparability across studies (Hector, 2011).
Each data collection method has limitations. Interviews are expensive and time consuming and require trained interviewers. Interviewers can introduce bias into a study in recording or interpreting information. Interviews and telephone surveys provide additional interaction between researchers and mothers and may influence the findings. Misclassifications in self-report questionnaires will occur with inappropriate terminology. Mailed questionnaires and online surveys may be less sensitive in detecting breastfeeding problems and concerns than interviews. A high attrition rate was noted in online surveys. Few reports were made for the reason for discontinuation of breastfeeding in questionnaires. Rates of breastfeeding initiation, duration, and exclusivity alone do not tell what caused breastfeeding discontinuation. This is a sensitive topic but having that information is essential to determine the causes of breastfeeding discontinuation. This should be asked during interviews. Overreporting of breastfeeding, particularly in the treatment group, might occur if mothers provided socially desirable answers, especially when they develop a good relationship with interventionists. Under- and/or overreporting of breastfeeding might occur if mothers do not keep an accurate record of their infant feeding.
Recommendations to Future Research
The feeding log is a method of recording breastfeeding events contingent on infant behavior and breastfeeding experiences that offer a tool that enables communication with healthcare professionals. Recall bias is avoided if the mother keeps feeding logs every 4 to 6 hours daily with minimum burden (Barnard & Eyres, 1979 ; Barnard & Thomas, 2014 ; Pollard, 2011 ; Wood et al., 2017). Feeding logs provide subjective data, but they increase validity and reliability when used with objective data of breastfeeding observations and interviews (Wood et al., 2016 , 2017).
The type of breastfeeding was collected as primary and/or secondary outcomes. However, outcomes of the modifiable causes of breastfeeding discontinuation, for example, maternal misattribution of infant behavior to perceived insufficient milk (Wood et al., 2016) are few. This is one of the methodological limitations. When choosing outcomes, researchers should first consider outcome variables attributed to the targets of the intervention, followed by outcomes of breastfeeding initiation, duration, and exclusivity.
This review has limitations. The limited number of qualified studies yielded by the constrained search strategy that only focused on English articles has the potential for selection bias. Variations in methodology across studies limited comparisons of the findings.
The evidence suggests that failure to assess both types and methods of breastfeeding is attributable, at least in part, to the effects of mediators and moderators of breastfeeding outcomes (e.g., a strong relationship between mother and infant). Poor quality of outcome measures: a 24-hour recall bias, misclassification of breastfeeding categories, lack of consistency in breastfeeding definitions, and few reports of the reason for breastfeeding discontinuation is attributed to the compromising effects of interventions. However, breastfeeding observation and feeding logs augmented by interviews appear to promote understanding between breastfeeding events contingent on infant behavior. Enhancement of the clinical application of outcomes measures will likely require considering outcomes measures that are attributed to the target of the intervention, followed by breastfeeding initiation, duration, or exclusivity as next steps in research.
The first author expresses appreciation to Dr. Frances M. Lewis for mentoring clinical trials. The authors express appreciation for Dr. Kathleen R. Helfrich-Miller for the assistance of manuscript preparation.
ITHS TL1 Multidisciplinary Pre-doctoral Clinical Research Training Program: NIH/National Center for Advancing Translational Sciences (TL1TR00422), Hester McLaws Nursing Scholarship from University of Washington School of Nursing, and Sigma Theta Tau International Psi-At-Large Chapter from University of Washington School of Nursing.
Suggested Clinical Implications
- The feeding log is a method of recording the types, methods, and patterns of breastfeeding contingent on infant behavior and breastfeeding experiences that offers a tool that enables mothers to communicate and gain support from healthcare professionals.
- Structured breastfeeding observations can help healthcare professionals objectively assess breastfeeding types and methods as well as mother–infant interactions.
- Interviews encourage breastfeeding mothers to talk about their breastfeeding experiences.
- These outcomes measures will separate breastfeeding from breast milk feeding via bottle that underscore the importance of health benefits, protective effects, and a stronger relationship between mother and infant.
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