The “gold standard” measurement of birth weight is the weight recorded in medical notes by clinical staff immediately after delivery. However, obtaining information from routine clinical records is not always feasible. Medical records may be missing, no longer available if the individual was born many years ago or have never existed (e.g. in some low-income settings). Recorded information may also not be available if the individual was born at home, or if delivery occurred outside the country of interest. Therefore, recovery rate of these records is generally low.
Recalled birth weights by mothers and/or fathers (maternal/parental recall) or adult self-reported birth weight are often the only source available. Birth weights recalled by mothers have been found to correlate highly with those found in routine clinical records. However, the accuracy of recall decreases with time since birth.
Selection of data source
Self-reported birth weight can be collected by in person or telephone interviews, by mail survey, electronic questionnaire (which can be self-administered) and the Internet.
Asking respondents to choose from birth categories rather than reporting birth weight in an open-ended format (e.g. “what was your child’s birth weight?” or “what was your birth weight?”) may increase reporting rates but reduce accuracy and result in loss of power and sensitivity to detect intra-categorical differences.
Example of categories in grams:
Units of measure
Ensure to clearly state the unit of the measurement e.g. metric units or imperial scale to avoid discrepancy in recording birth weight. Incorrect conversion may lead to errors. Use units appropriate for the population or ethnic group of interest.
When studies rely on self-reported measures, a validation study could help to quantify the potential errors and to calculate correction factors. Ideally, medical records should be used to validate participants’ reporting of birth weight.
Life-course epidemiological studies, investigating the relationship between factors at birth and morbidity and mortality in adult life, often obtain early life data (e.g. birth weight) through self-reported information.
Corrections of self-reported birth weight may be required to derive valid estimates. Heaping or spikiness of reported birth weights, in which respondents show a tendency to round birth weight information to the nearest digit (e.g. 2500 g instead to 2475 g) is one issue which can be addressed by correction. This phenomenon tends to affect the classification of infants in the low birth weight category and it has been frequently observed in retrospective birth weight surveys in less developed countries. To deal with the heaping issue, Boerma et al 1996  and Blanc & Wardlaw 2005  proposed an adjustment procedure on low birth weight estimates in less developed countries.
If birth weight is used as an independent variable, self-reported birth weights may have to be adjusted using birth records when available. The recalled method can also be controlled for, in regression analysis aimed at predicting birth weight outcomes.
Key characteristics of self-reported birth weight methods are outlined in Table A.2.1
Table A.2.1 Characteristics of subjective birth weight methods.
|Number of participants
|Researcher burden of data collection
|Researcher burden of coding and data analysis
|Risk of reactivity bias
|Risk of recall bias
|Risk of social desirability bias
|Risk of observer bias
|Suitability for field use
|Participant literacy required
|Yes, if self-administered
Considerations relating to the use of subjective birth weight methods in specific populations are described in Table A.2.2.
Table A.2.2 Use of subjective birth weight methods in different populations.
|Infancy and lactation
|Toddlers and young children
|The reliability of self-reported birth weight also decreases in older individuals as they may no longer have access to their mother’s recall, or are unable to accurately remember reports from their mother.
|The accuracy of recalled birth weight varies according to maternal ethnicity and cultural differences (e.g. desirability of large vs small babies could affect reporting).
|The accuracy of recalled birth weight varies according to socioeconomic status and parity.
The ability to recall birth weight increases with parental education, but decreases with the birth order of the child (first births are recalled more accurately than subsequent births).
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