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On the importance--and the unimportance--of birthweight
1Epidemiology Branch, National Institute of Environmental Health Sciences, Durham NC 27709, USA. WILCOX@NIEHS.NIH.GOV
Insights
Birthweight is often misunderstood in epidemiology. New evidence suggests the link between birthweight and health outcomes may not be causal, challenging common analytical assumptions.
Area of Science:
- Epidemiology
- Public Health
- Biostatistics
Background:
- Birthweight is a widely used epidemiological variable, strongly associated with infant mortality, childhood development, and adult disease risk.
- Epidemiological studies frequently assume birthweight is on the causal pathway to these health outcomes.
- This assumption leads to birthweight being used to explain variations in infant mortality and as an intermediate health endpoint.
Purpose of the Study:
- To critically evaluate the causal assumptions linking birthweight to health outcomes.
- To assess the reliability and potential biases of analytical methods that assume causality.
- To re-examine the utility and interpretation of birthweight categories, particularly 'low birthweight'.
Main Methods:
- Review and critique of epidemiological analytical methods that treat birthweight causally.
- Examination of the evidence supporting or refuting a direct causal link between birthweight and subsequent health.
- Exploration of alternative interpretations for the observed associations between birthweight and health outcomes.
Main Results:
- Evidence suggests that the association between birthweight and health outcomes may not be causal.
- Analytical methods assuming causality are potentially unreliable and can introduce bias.
- The 'low birthweight' category is often uninformative and lacks strong justification.
Conclusions:
- The common assumption of causality between birthweight and health outcomes is questionable.
- Standard epidemiological approaches to birthweight may be flawed.
- While direct causality is uncertain, birthweight distributions may still reflect underlying biological mechanisms relevant to human health, particularly for estimating preterm birth rates with appropriate methods.
Abstract:
Birthweight is one of the most accessible and most misunderstood variables in epidemiology. A baby's weight at birth is strongly associated with mortality risk during the first year and, to a lesser degree, with developmental problems in childhood and the risk of various diseases in adulthood. Epidemiological analyses often regard birthweight as on the causal pathway to these health outcomes. Under this assumption of causality, birthweight is used to explain variations in infant mortality and later morbidity, and is also used as an intermediate health endpoint in itself. Evidence presented here suggests the link between birthweight and health outcomes may not be causal. Methods of analysis that assume causality are unreliable at best, and biased at worst. The category of 'low birthweight' in particular is uninformative and seldom justified. The main utility of the birthweight distribution is to provide an estimate of the proportion of small preterm births in a population (although even this requires special analytical methods). While the ordinary approaches to birthweight are not well grounded, the links between birthweight and a range of health outcomes may nonetheless reflect the workings of biological mechanisms with implications for human health.