Each year, approximately 20 percent of the almost 4 million infants in the United States are born at the low and high extremes of fetal growth. Although most low-birthweight infants are preterm, approximately 3 percent are term. In 2006, 8.3 percent of infants weighed less than 2500 g at birth, whereas 7.8 percent weighed more than 4000 g. The proportion of those < 2500 g has increased by 22 percent since 1984 and by 8 percent since 2000. At the same time, the incidence of macrosomia—defined as birthweight > 4000 g—continues to decline as the distribution has shifted toward lower weights