WebMD Medical News
Louise Chang, MD
Jan. 21, 2010 -- Birth weights of full-term babies in the United States
decreased from 1990 to 2005, a new study says.
Researchers at the Harvard Pilgrim Health Care Institute’s Department of
Population Medicine, an affiliate of Harvard Medical School, and Boston
University published their findings in the February 2010 issue of Obstetrics
The researchers say their findings are surprising because previous studies
have indicated that birth weights have been increasing over the past half
But that’s not what’s been happening in the past 15 years, at least, says
co-author Emily Oken, MD, MPH, an assistant professor of medicine at Harvard
She says her research team expected to find that birth weights were still
increasing, partly because of increasing age of mothers giving birth and
decreased rates of smoking.
Instead, the researchers say they found that birth weights of full-term
babies decreased by an average of 1.83 ounces between 1990 and 2005, and that
decreases were especially notable after 1995.
The researchers analyzed data on birth weight, maternal and neonatal
characteristics, obstetric care, and other trends from the National Center for
Health Statistics, looking at records of 36.8 million single-birth babies born
full term in the United States in the 15-year period ending in 2005.
The authors say they teased out factors such as age of mothers, race and
ethnicity, education level, marital status, tobacco use, and the amount of
weight the women had gained during pregnancy.
The researchers also took into account the risk of hypertension and use of
procedures such as induction of labor and cesarean delivery.
Birth weights were even lower for babies born to women considered to be at
low risk of having small babies, the researchers say.
Mothers who were white, well educated, married, didn’t smoke, and received
early prenatal care had babies weighing an average of 2.78 ounces less at birth
in 2005 compared to 1990, the authors note.
Average pregnancy time among full-term births also dropped by more than two
days, the researchers say. The researchers note that factors such as the
decreased pregnancy time and increased use of C-sections for delivery do not
account for the declines in newborn weight.
The decline in birth weights may represent a reversal of previous increases
and needs further investigation, the researchers say.
The researchers say additional studies may identify other factors that might
contribute to lower birth weight, such as trends in diets of mothers, stress,
physical activity, and exposure to environmental toxins.
“There’s still a lot we don’t know about the causes of low birth weight,”
Oken says in the news release.
SOURCES:News release, Harvard Medical School.Donahue, S. Obstetrics & Gynecology, February 2010: vol 115, no
2, Part 1.
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