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The
Wilcox-Russell Hypothesis
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Broader
implications
Much public health emphasis has been put on LBW as a useful endpoint
for studying perinatal health. The analysis presented here suggests
that the three basic assumptions about LBW are not valid (see
a short history of low birth weight).
- "LBW is a cause of infant mortality." Not directly.
Preterm delivery IS a cause of infant mortality, but preterm
births are an inconstant fraction of LBW. As researchers recognized
in the 1960s and 1970s, LBW is not a good surrogate for preterm
delivery. The remaining portion of LBW is the lower tail of
the predominant distribution, which exists in all populations
and is not a "cause" of infant mortality.
- "The percent of LBW in any group is an indicator of infant
risk." It's not a good risk indicator, because it is muddled
by changes in the predominant distribution. It is true that
some factors which increase infant risk also reduce birth weight
(smoking or social class are good examples). But changes in
birthweight are not causally related to mortality. Changes in
the position of the predominant distribution do not directly
change risk. This makes the % of LBW unreliable in its relation
to infant risk.
- "LBW is a target for interventions to improve infant survival."
Interventions to decrease preterm delivery are warranted. However,
since birthweight is not on the causal pathway to infant mortality,
changes in birthweight cannot directly change mortality. Of
course, changes to risk factors (such as maternal smoking) may
change birth weight and also improve infant outcome. But the
improvement in survival is not caused by change in birth weight.
A BIRTH WEIGHT ANALYSIS PROGRAM is available
on this website for you to use on your birth weight data. The
program estimates the residual distribution, and also the mean
and standard deviation of the predominant distribution (needed
to plot weight-specific mortality rates on a z-scale). Thus, the
program provides the basis for analysis of both the birth weight
distribution and birth weight-specific mortality.
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