Analysis of 7776 singleton births defined a cohort of babies with birthweight below the 10th percentile after adjusting for gestational age and sex. The relative risk of a baby being small for gestational age in respect to a number of factors, such as parental anthropometry, demographic factors, behavior patterns (tobacco, cannabis, alcohol, and caffeine consumption), maternal pathology, and fetal abnormality, was calculated. The highest relative risks are associated with severe antepartum hemorrhage, severe pre-eclampsia, and severe fetal abnormality. As these are relatively rare events, a more accurate calculation of overall risk to the population as opposed to the individual can be obtained by studying the percent attributable risk of each of the factors. This demonstrates that maternal tobacco consumption is the major environmental risk factor in our population

Andersen, M J

Keeping, J D

Morrison, J

Najman, J M

Williams, G M

Environmental Health Perspectives

English

PubMed

J. Morrison

G. M. Williams

J. M. Najman

M. J. Andersen

J. D. Keeping

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Birthweight below the Tenth Percentile: The Relative and Attributable Risks of Maternal Tobacco Consumption and Other Factors

Analysis of 7776 singleton births defined a cohort of babies with birthweight below the 10th percentile after adjusting for gestational age and sex. The relative risk of a baby being small for gestational age in respect to a number of factors, such as parental anthropometry, demographic factors, behavior patterns (tobacco, cannabis, alcohol, and caffeine consumption), maternal pathology, and fetal abnormality, was calculated The highest relative risks are associated with severe antepartum hemorrhage, severe pre-eclampsia, and severe fetal abnormality. As these are relatively rare events, a more accurate calculation of overall risk to the population as opposed to the individual can be obtained by studying the percent attributable risk of each of the factors. This demonstrates that maternal tobacco consumption is the major environmental risk factor in our population

Morrison, J.

Williams, G. M.

Najman, J. M.

Andersen, M. J.

Keeping, J. D.

University of Queensland eSpace

Environmental Health Perspectives Supplements 101(Supp 3): 275-277 (1993) 
Birthweight Below the Tenth Percentile: The Relative and Attributable 
Risks of Maternal Tobacco Consumption and Other Factors 
 
BY J. Morrison,1 G. M. Williams,1 J. M. Najman,1 M. J. Andersen,1 and J. D. 
Keeping1
 
Analysis of 7776 singleton births defined a cohort of babies with birthweight below 
the 10th percentile after adjusting for gestational age and sex. The relative risk of a 
baby being small for gestational age in respect to a number of factors, such as parental 
anthropometry, demographic factors, behavior patterns (tobacco, cannabis, alcohol, 
and caffeine consumption), maternal pathology, and fetal abnormality, was calculated 
The highest relative risks are associated with severe antepartum hemorrhage, severe 
pre-eclampsia, and severe fetal abnormality. As these are relatively rare events, a 
more accurate calculation of overall risk to the population as opposed to the 
individual can be obtained by studying the percent attributable risk of each of the 
factors. This demonstrates that maternal tobacco consumption is the major 
environmental risk factor in our population. 
 
Introduction 
The phrase “birthweight below the tenth percentile” is internationally used to define a 
particular group of babies who have significantly increased perinatal mortality and 
morbidity rates, although these rates vary between different populations. A number of 
synonyms are also used to describe this group. These include ‘intrauterine growth 
retardation” as defined by the World Health Organization, “small for gestational age,” 
where specific allowance is made for gestation, and more precisely “small for 
gestational age and sex” (SGAS), where the effect of gender on birthweight is also 
taken into account, and we use SGAS in this paper. 
 
It is obvious that any baby’s birthweight is a balance between two major factors. One 
is the genetic potential derived from parents, particularly the mother. The other is 
environment of the developing fetus, which can be modified by a vast variety of 
factors and may be integral to the mother or fetus or the external environment of both. 
This paper examines a number of factors and their associations with SGAS babies. 
 
Survey Methods 
The data are derived from a prospective, longitudinal, collaborative study of 
pregnancy at the University of Queensland in which 8556 consecutive antenatal 
patients were enrolled and information gathered via questionnaires at the first clinic 
visit, 3 days after delivery, and 6 months after delivery, as well as a comprehensive 
clinical evaluation of the mother and baby. These data sets consisted of 117, 103, 103, 
and 97, items, respectively. The details of methodology and compliance rates have 
been previously published1,2. 
 
Statistical Methods 
The babies were ranked by birthweight within each gestational age and sex category. 
Those within the first decile were defined as SGAS—the dependent variable. Logistic 
regression was used to examine a number of prediction models (loosely grouped as 
                                                 
1University of Queensland, Brisbane, Australia. 
Address reprint requests to J. Morrison. University of Queensland, Brisbane, Australia. 
Environmental Health Perspectives Supplements 101(Supp 3): 275-277 (1993) 
parental anthropometry, demography, lifestyle, pregnancy pathology, and fetal 
pathology) for SGAS, and the odds ratio and 95% confidence limits were estimated. 
The significance of individual factors was assessed by the likelihood ratio statistic, 
both for trend and variation. 
 
Results 
Parental Anthropometry. Low maternal height, low maternal weight (as body mass 
index) and low weight gain in pregnancy were all significantly associated with SGAS, 
as also was low paternal height but not low paternal weight, These results confirm our 
previous findings that paternal height has a significant effect on birthweight3. 
Demography. Maternal age was nonsignificant, but maternal race and parity (in 
descending birth order) were significantly associated with SGAS. Family income was 
not an important factor in SGAS, a point we have previously made with regard to 
socioeconomic status, in respect to birthweight in our Australian population, and this 
is probably a reflection of the general affluence of all sections of the community4. 
Lifestyle. Cannabis/caffeine and alcohol consumption was nonsignificant. Rarely or 
never eating breakfast was, however, significantly associated with SGAS and 
presumably reflects a variety of lifestyle variables. Cigarette smoking was highly 
significant, and the risk ratio for SGAS rises progressively with increased cigarette 
consumption. 
Pregnancy Pathology. The major diseases of pregnancy — essential hypertension, 
pre-eclamptic toxemia, and antepartum hemorrhage — had no significant association 
with SGAS in our population, probably due to the small numbers and active 
obstetrical management. 
Fetal Pathology—severe developmental defects and, more particularly, chromosomal 
abnormalities—had significantly increased risk ratios for SGAS. 
 
Discussion 
Figure 1 summarizes the risk ratio of a baby being SGAS in relation to the previous 
factors and demonstrates a descending order from major fetal abnormality through 
cigarette smoking, antepartum hemorrhage, various aspects of maternal 
anthropometry, and a series of relatively minor variables. However, these data only 
show the risk of an individual baby being SGAS where these factors pertain. What we 
really need to know is the epidemiological or “global” effect of these factors in 
relation to SGAS, an aspect that does not appear to have been previously considered 
We have therefore used a formula derived from Kahn and Sempos5, which combines 
both the effect and the frequency of the variable, to derive the population percent 
attributable risk. These results (Fig. 2) clearly demonstrate that parental 
anthropometry and cigarette smoking are by far the most common factors associated 
with SGAS. 
 
In conclusion, in our population, maternal cigarette smoking is the major preventable 
factor in SGAS babies and as such is a major cause of perinatal mortality and 
morbidity. 
 
Environmental Health Perspectives Supplements 101(Supp 3): 275-277 (1993) 
Figure 1: Average risk ratio of small for gestational age and sex infants 
 
 
Figure 2: Percent attributable risk of small for gestational age and sex infants 
 
 
REFERENCES 
1. Keeping, J. D., Najman, J. M Morrison, J., Western, J. S., Andersen, M. J., and 
Williams, G. M. A prospective longitudinal study of social, psychological and 
Environmental Health Perspectives Supplements 101(Supp 3): 275-277 (1993) 
obstetric factors in pregnancy: response rates and demographic characteristics of the 
8556 respondents. Br. J. Obstet, Gynaecol. 99: 289-297 (1989). 
2. Najman, ,J. M, Morrison, J., Williams, G. M., Keeping J. D,, and Andersen, M. J. 
Unemployment and reproductive outcome. An Australian study. Br. .J. Obstet. 
Gynaecol. 99: 308—313 (1989). 
3. Morrison, .1., Williams, G. M Najman, J. M, and Andersen, M. J. ‘The influence of 
paternal height and weight on birthweight. Aust. N.Z. 1. Obstet. Gynaecol. 3l:(2): 
114-416 (1991). 
4. Morrison, .1., Najman, J. M., Williams, G. M., Keeping J. D., and Andersen, M. .1. 
Socioeconomic status and pregnancy outcome. An Australian study. Br. J. Obstet. 
Gynuecol. 99: 298—307 (1989). 
5. Kahn, H. A., and Sempos, L. T. Statistical Methods in Epidemiology. Oxford 
University Press, Oxford (1989). 


Birthweight Below the Tenth Percentile: The Relative and Attributable Risks of Maternal Tobacco Consumption and Other Factors

A prospective longitudinal study of social, psychological and obstetric factors in pregnancy: response rates and demographic characteristics of the 8556 respondents.

Socio-economic status and pregnancy outcome. An Australian study.

Statistical Methods in Epidemiology.

The influence ofpaternal height and weight on birthweight.

Unemployment and reproductive outcome. An Australian study.

http://espace.library.uq.edu.au/view/UQ:7923/musp_93_Birthwei.pdf

Birthweight below the tenth percentile: the relative and attributable risks of maternal tobacco consumption and other factors.

Birthweight below the tenth percentile: the relative and attributable risks of maternal tobacco consumption and other factors.

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