A systematic review was conducted in January and February 2007 using the key words "(obesity OR BMI) AND (breastf* OR lactation OR lactating)". The following databases were searched (all languages, from the start of the database):
• Medline via PubMed (8 February 2007) 767 items, 24 were relevant;
• CINAHL (Cumulative Index to Nursing & Allied Health Literature) (8 February 2007) 103 items, 11 relevant (1 additional);
• Australian Breastfeeding Association's Lactation Resource Centre database was searched for 'obesity' (21 January 2007); 172 items, 10 relevant (3 additional).
Most of the articles found in the databases were about the relationship between breastfeeding and childhood
obesity and were therefore not relevant to this review. All papers related to maternal overweight and obesity and infant feeding were located and included if appropriate. Papers that were case studies, clinical papers or reviews were not included in the tables [19
]. Research studies were also identified from the reference lists of included articles, and the authors' literature collection was hand-searched (n = 2230; nine additional studies). Papers which had cited the original Rutishauser and Carlin study were identified (n = 6), but no new papers were revealed. The total number of research articles included in this review is 27.
Five papers were excluded as they did not define overweight or obesity (e.g. presented body mass index (BMI, kg/m2
) as a continuous variable [29
] or used vague terms, such as "heavy before becoming pregnant" [33
]). However, as there were very few studies on women's infant feeding intention and maternal overweight and obesity, a study which examined women's "weight concerns" was included [34
] as this information was also relevant. Thus, 22 papers are included in the tables (27 less 5).
Most studies based their evaluation of maternal obesity on the World Health Organization (WHO) definition of obesity [35
]: normal weight BMI<25, overweight BMI 25 ≤ 30, obese BMI > 30, or the US Institute of Medicine (IOM) definition [36
]: underweight/normal weight BMI < 26.1, overweight BMI 26.1 – 29.0, obese BMI > 29.0.
The papers have been grouped according to the content of the study and presented in tables alphabetically by the first author. As women's infant feeding intention has been found to be the strongest single predictor of breastfeeding behaviour [37
], all studies reporting infant feeding intention and maternal obesity, including those with "vague" definitions, have been included in Table (Included studies on maternal obesity and women's infant feeding intentions).
Included studies on maternal obesity and women's infant feeding intentions
The indicators suggested for monitoring breastfeeding have been described as:
• Initiation (the infant's first intake of breast milk)
• Intensity (the degree of exclusiveness of breast milk as the source of nourishment for the infant) and
• Total duration (the total length of time that an infant receives any breast milk at all [38
Authorities recommend that breastfeeding initiation is defined as "ever breastfed/ever given breast milk" so that infants who only went to the breast once or only received expressed breast milk are included [38
]. However, research studies have used a range of definitions, including breastfeeding at hospital discharge [12
], breastfeeding at four days [39
], feeding in last five feeds in hospital [40
]. Table (Included studies on maternal obesity and initiation of breastfeeding) includes the studies which have reported breastfeeding initiation (however defined), and the authors' definition when it varies from "ever breastfed". Where the authors have not presented an odds ratio (OR) for breastfeeding initiation we have calculated one using data from their publication (labelled as "our calculation of overweight/obese women not initiating breastfeeding"). These are unadjusted odds ratios as we did not have the data to adjust for potential confounding factors such as income or method of birth. We have not performed a meta-analysis as the definitions of initiation are inconsistent.
Included studies on maternal obesity and initiation of breastfeeding
It has been hypothesised that the onset of lactation occurs later in obese women than other women, therefore all studies which have investigated this are listed in Table (Included studies on maternal obesity and delayed onset of lactation).
Included studies on maternal obesity and delayed onset of lactation
The final table (Table . Included studies on maternal obesity and duration of breastfeeding) includes studies which report total duration of breastfeeding and, where reported, exclusive breastfeeding; multivariate analysis has been included when this has been conducted.
Included studies on maternal obesity and duration of breastfeeding