There is a strong association between very low birth weight (due to preterm delivery or extreme fetal growth restriction) and infant mortality that decreases as birth weight increases until it reaches about 4,500 g, when there is a slight increase in infant mortality due to problems associated with macrosomia (Mathews and MacDorman, 2007). Although rates of infant mortality have decreased over time, the reverse J-shape of this relationship has not changed.
Between 1990 and 2005, the proportion of small infants increased and the proportion of large infants decreased (Figure 2-23). This downward shift in the overall distribution of birth weight is attributable in part to an increase in multiple births, but the pattern is similar for singleton births. Other possible explanations for these trends in birth weight include a greater prevalence of older mothers, who tend to have more complications of pregnancy, as well as increased use of assisted reproductive technology and obstetrical procedures, including labor induction and elective cesarean deliveries.
Rates for low birth weight and very low birth weight increased in the United States between 1990 and 2005, when the overall rate of low birth weight among singletons was 6.41 percent and the overall rate of very low birth weight was 1.14 percent. The lowest rates of low birth weight are among Hispanic and white infants, the highest among black infants; Native American, and Asian/Pacific Islander infants fall in between (Figure 2-24). Low birth weight also varies by maternal age, with greater prevalence among women < 20 and > 40 years of age (Martin et al., 2008).