Trends in birth weight and gestational length among singleton term births in the United States: 1990-2005

Abstract

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.

Level of evidence: III.

Figures

Fig. 1

Trends in birth weight from 1990 to 2005 among all 36,827,828 singleton births born at 37 to 41 completed weeks of gestation and by gestational age at birth among a low-risk subgroup of 502,716 singleton neonates born to mothers of non-Hispanic white race/ethnicity with 13 or more years of education and of married status, who received prenatal care in the first trimester, were non-smokers, had no pregnancy complications, delivered vaginally, did not have labor induced, had a prenatal ultrasound examination, and gained 26 to 35 pounds during pregnancy. Donahue. Trends in Birth Weight in the United States. Obstet Gynecol 2010.

Fig. 2

Trends in prevalence of small-for-gestational age (SGA) and large-for-gestational age (LGA) births from 1990 to 2005 among 502,716 singletons born at 37 to 41 completed weeks of gestation to low-risk mothers aged 25 to 29 years of non-Hispanic white race/ethnicity, 13 or more years of education, and of married status, who received prenatal care in the first trimester, were non-smokers, had no pregnancy complications, delivered vaginally, did not have labor induced, had a prenatal ultrasound examination, and gained 26 to 35 pounds during pregnancy. Donahue. Trends in Birth Weight in the United States. Obstet Gynecol 2010.

Fig. 3

Trends in length of gestation from 1990 to 2005, according to induction of labor status and route of delivery, among 36,827,828 singletons born at 37 to 41 completed weeks of gestation, and among a low-risk subgroup of 502,716 singleton neonates born to mothers aged 25 to 29 years, of non-Hispanic white race/ethnicity, with 13 or more years of education, of married status, who received prenatal care in the first trimester, were nonsmokers, had no pregnancy complications, delivered vaginally, did not have labor induced, had a prenatal ultrasound examination, and gained 26 to 35 pounds during pregnancy. Donahue. Trends in Birth Weight in the United States. Obstet Gynecol 2010.