Secular Trends in Birthweight

Abstract

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.

The World Health Organization Fetal Growth Charts: A Multinational Longitudinal Study of Ultrasound Biometric Measurements and Estimated Fetal Weight

“…Humans adapt across generations to local conditions, and fetal development adds an important adaptive refinement for the next generation. Secular changes in birthweight and child growth patterns have been shown to accompany social changes [42,43]. Fetal growth charts may thus need to be adjusted to fit the diversity of individuals and populations if they are to be of the greatest clinical utility.…”

Trends in prevalence of gestational diabetes and perinatal outcomes in Catalonia, Spain, 2006 to 2015: the Diagestcat Study

“…With regard to birth weight outcomes, the rate of LGA babies remained stable, whereas macrosomia rates declined throughout the study period in women with GDM. These findings suggest that the observed reduction in macrosomia rates might not reflect a true decreasing trend in birth weight but could be associated, as described in previous studies, with a reduction in gestation length owing to labour induction or prematurity. The results obtained in women with GDM differ partly from those observed in women without DM.…”

The positive association of infant weight gain with adulthood body mass index has strengthened over time in the Fels Longitudinal Study

“…Prenatal growth rate, indexed by birth weight, has generally increased during the obesity epidemic , likely because of higher levels of maternal obesity . Given that higher birth weight is positively related to risk of obesity later in life , it is possible that the secular trend towards increasing birth weight may have contributed to the development of the obesity epidemic.…”