Trends in Birth Weight and Gestational Length Among Singleton Term Births in the United States 1990-2005

OBJECTIVE: To estimate changes over time in birth weight for gestational age and in gestational length among term singleton neonates born from 1990 to 2005. METHODS: We used data from the U.S. National Center for Health Statistics for 36,827,828 singleton neonates born at 37-41 weeks of gestation, 1990-2005. We examined trends in birth weight, birth weight for gestational age, large and small for gestational age, and gestational length in the overall population and in a low-risk subgroup defined by maternal age, race or ethnicity, education, marital status, smoking, gestational weight gain, delivery route, and obstetric care characteristics. RESULTS: In 2005, compared with 1990, we observed decreases in birth weight (-52 g in the overall population, -79 g in a homogenous low-risk subgroup) and large for gestational age birth (-1.4% overall, -2.2% in the homogenous subgroup) that were steeper after 1999 and persisted in regression analyses adjusted for maternal and neonate characteristics, gestational length, cesarean delivery, and induction of labor. Decreases in mean gestational length (-0.34 weeks overall) were similar regardless of route of delivery or induction of labor. CONCLUSION: Recent decreases in fetal growth among U.S., term, singleton neonates were not explained by trends in maternal and neonatal characteristics, changes in obstetric practices, or concurrent decreases in gestational length.