This review of common risk factors for low birthweight emphasizes the usefulness of
examining the entire distribution of birthweight. Of the factors we examined, only
short gestational age seemed to affect the low end of the birthweight distribution
in the form of skewness. Most factors, such as maternal race, infant sex, plurality,
altitude, education, and smoking seem to affect the entire birthweight distribution,
indicating a generalized effect. With the exceptions of race, infant sex, parity,
and altitude, these factors seemed to have similar associations with both low birthweight
and infant mortality. However, only the effects of race and sex on mortality have
been repeatedly studied in detail for different combinations of gestational age and
birthweight. A few of the factors examined, notably infant sex and parity, have opposite
associations with birthweight and infant mortality. Female infants and firstborn infants
have lower birthweights than their counterparts, but are more likely to survive. For
factors that significantly affect the birthweight distribution, but do not affect
mortality equally across the birthweight distribution, the development and use of
population-based standards may result in less misclassification of IUGR. Separate
standards by infant sex, altitude, and perhaps race may lead to more accurate classification
of intrauterine growth. Last, the majority of risk factors have differential effects
on birthweight depending on the level of the associated factors. For example, low
maternal age and low prepregnancy BMI are associated with both increased risk of low
birthweight and poor infant survival. Older maternal age and high prepregnancy BMI
are associated with reduced risk of low birthweight, but with increased risk of infant
mortality. One possible explanation is that young maternal age and low prepregnancy
BMI are associated with adverse behavioral risk factors such as cigarette smoking,
whereas increased age and high prepregnancy BMI are associated with gestational diabetes,
multiparity, and genetic defects. It is possible that the greater variation in birthweight
at the high end of the scale is indicative of increased risk of mortality. Thus, higher
birthweight does not always equal better birth outcomes.