The influence of fetal and maternal factors on the distribution of birthweight.

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.