Low birth weight paradox
Encyclopedia
The low birth weight paradox is an apparently paradox
ical observation relating to the birth weight
s and mortality of children born to tobacco smoking
mothers. Low birth weight
children born to smoking mothers have a lower infant mortality
rate than the low birth weight children of non-smokers. The same is true of children born to poor
parents, and of children born at high altitude; these are all examples of Simpson's paradox
.
) have been classified as having low birth weight. In a given population, low birth weight babies have a significantly higher mortality
rate than others; thus, populations with a higher rate of low birth weights typically also have higher rates of child mortality than other populations.
Based on prior research, the children of smoking mothers are more likely to be of low birth weight than children of non-smoking mothers. Thus, by extension the child mortality rate should be higher among children of smoking mothers. So it is a surprising real-world observation that low birth weight babies of smoking mothers have a lower child mortality than low birth weight babies of non-smokers.
by uncovering a lurking variable
between smoking and the two key variables: birth weight and risk of mortality. Both are acted on independently when the mother of the child smokes — birth weight is lowered and the risk of mortality increases.
The birth weight distribution
for children of smoking mothers is shifted to lower weights by their mothers' actions. Therefore, otherwise healthy babies (who would weigh more if it were not for the fact their mother smoked) are born underweight. They have a lower mortality rate than children who have other medical reasons why they are born underweight, regardless of the fact their mother does not smoke.
In short, smoking may be harmful in that it contributes to low birth weight, but other causes of low birth weight are generally more harmful only with regards to their weight.
by smoking, via stratification or multivariable regression modelling to statistical control for smoking, then one finds that the association between birth weight and mortality may be attenuated towards the null. Nevertheless, most epidemiologic studies of birth weight and mortality have controlled for maternal smoking, and the adjusted results, although attenuated after adjusting for smoking, still indicated a significant association.
Additional support for the hypothesis that birth weight and mortality can be acted on independently came from the analysis of birth data from Colorado
: compared with the birth weight distribution in the U.S.
as a whole, the distribution curve in Colorado is also shifted to lower weights. The overall child mortality of Colorado children is the same as that for US children however, and if one corrects for the lower weights as above, one finds that babies of a given (corrected) weight are just as likely to die, whether they are from Colorado or not. The likely explanation here is that the higher altitude
of Colorado affects birth weight, but not mortality.
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...
ical observation relating to the birth weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
s and mortality of children born to tobacco smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...
mothers. Low birth weight
Low birth weight
Low birth weight is defined as a birth weight of a liveborn infant of less than 2,500 g. regardless of gestational age-Causes:LBW is either the result of preterm birth or of the infant being small for gestational age , or a combination of...
children born to smoking mothers have a lower infant mortality
Infant mortality
Infant mortality is defined as the number of infant deaths per 1000 live births. Traditionally, the most common cause worldwide was dehydration from diarrhea. However, the spreading information about Oral Re-hydration Solution to mothers around the world has decreased the rate of children dying...
rate than the low birth weight children of non-smokers. The same is true of children born to poor
Poverty
Poverty is the lack of a certain amount of material possessions or money. Absolute poverty or destitution is inability to afford basic human needs, which commonly includes clean and fresh water, nutrition, health care, education, clothing and shelter. About 1.7 billion people are estimated to live...
parents, and of children born at high altitude; these are all examples of Simpson's paradox
Simpson's paradox
In probability and statistics, Simpson's paradox is a paradox in which a correlation present in different groups is reversed when the groups are combined. This result is often encountered in social-science and medical-science statistics, and it occurs when frequencydata are hastily given causal...
.
History
Traditionally, babies weighing less than a certain amount (which varies between countriesCountry
A country is a region legally identified as a distinct entity in political geography. A country may be an independent sovereign state or one that is occupied by another state, as a non-sovereign or formerly sovereign political division, or a geographic region associated with a previously...
) have been classified as having low birth weight. In a given population, low birth weight babies have a significantly higher mortality
Infant mortality
Infant mortality is defined as the number of infant deaths per 1000 live births. Traditionally, the most common cause worldwide was dehydration from diarrhea. However, the spreading information about Oral Re-hydration Solution to mothers around the world has decreased the rate of children dying...
rate than others; thus, populations with a higher rate of low birth weights typically also have higher rates of child mortality than other populations.
Based on prior research, the children of smoking mothers are more likely to be of low birth weight than children of non-smoking mothers. Thus, by extension the child mortality rate should be higher among children of smoking mothers. So it is a surprising real-world observation that low birth weight babies of smoking mothers have a lower child mortality than low birth weight babies of non-smokers.
Explanation
At first sight these findings seemed to suggest that, at least for some babies, having a smoking mother might be beneficial to one's health. However the paradox can be explained statisticallyStatistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
by uncovering a lurking variable
Lurking variable
In statistics, a confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable...
between smoking and the two key variables: birth weight and risk of mortality. Both are acted on independently when the mother of the child smokes — birth weight is lowered and the risk of mortality increases.
The birth weight distribution
Frequency distribution
In statistics, a frequency distribution is an arrangement of the values that one or more variables take in a sample. Each entry in the table contains the frequency or count of the occurrences of values within a particular group or interval, and in this way, the table summarizes the distribution of...
for children of smoking mothers is shifted to lower weights by their mothers' actions. Therefore, otherwise healthy babies (who would weigh more if it were not for the fact their mother smoked) are born underweight. They have a lower mortality rate than children who have other medical reasons why they are born underweight, regardless of the fact their mother does not smoke.
In short, smoking may be harmful in that it contributes to low birth weight, but other causes of low birth weight are generally more harmful only with regards to their weight.
Evidence
If one corrects and adjusts for the confoundingConfounding
In statistics, a confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable...
by smoking, via stratification or multivariable regression modelling to statistical control for smoking, then one finds that the association between birth weight and mortality may be attenuated towards the null. Nevertheless, most epidemiologic studies of birth weight and mortality have controlled for maternal smoking, and the adjusted results, although attenuated after adjusting for smoking, still indicated a significant association.
Additional support for the hypothesis that birth weight and mortality can be acted on independently came from the analysis of birth data from Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
: compared with the birth weight distribution in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
as a whole, the distribution curve in Colorado is also shifted to lower weights. The overall child mortality of Colorado children is the same as that for US children however, and if one corrects for the lower weights as above, one finds that babies of a given (corrected) weight are just as likely to die, whether they are from Colorado or not. The likely explanation here is that the higher altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
of Colorado affects birth weight, but not mortality.
See also
- ConfoundingConfoundingIn statistics, a confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable...
- EpidemiologyEpidemiologyEpidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
- Epidemiologic methods
- Simpson's paradoxSimpson's paradoxIn probability and statistics, Simpson's paradox is a paradox in which a correlation present in different groups is reversed when the groups are combined. This result is often encountered in social-science and medical-science statistics, and it occurs when frequencydata are hastily given causal...
, of which the Low birth weight paradox is an example - Mexican paradoxMexican paradoxThe Mexican paradox is the observation that the Mexican people exhibit a surprisingly low incidence of low birth mass, contrary to what would be expected from their socioeconomic status...
, not directly related, but also involving low birth weights
External links
- The Analysis of Birthweight, by Allen Wilcox