Secular trends in birthweight.

From the mid- to late 20th century, average birthweight increased in many countries, including the United States. However, more recent data now suggest that mean birthweight has begun to decline. The most recent US data indicate that in 2008, compared with 1990, about half as many babies were macrosomic at birth (≥5,000 g), whereas there was a 17% increase in low birthweight (<2,500 g). Part of the observed decline in birthweight likely relates to decreases in gestation length and corresponding increases in rates of preterm and early term birth over the past several decades. However, available data suggest that fetal growth has also declined independent of gestational age at birth. Since 2000, rates of small for gestational age have increased, whereas rates of large for gestational age have decreased. Declines in birthweight and macrosomia are most likely largely explained by decreases in gestation length, itself caused by obstetric interventions, especially induction of labor and to a lesser extent elective cesarean delivery. However, it appears that fetal growth is also declining, at least in some settings, independent of gestation length. Reasons for this decline are as of yet unexplained and merit further investigation.