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Health & Pregnancy

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Lower Birth Weights for Full-Term Babies

Full-Term Birth Weights Decreased on Average 1.83 Ounces From 1990 to 2005, Study Finds
WebMD Health News
Reviewed by 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 & Gynecology.

The researchers say their findings are surprising because previous studies have indicated that birth weights have been increasing over the past half century.

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 Medical School.

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.

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