Lower Birth Weights for Full-Term Babies

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Jan. 21, 2010 — Birth weights of full-term babies in the United Statesdecreased from 1990 to 2005, a new study says.

Researchers at the Harvard Pilgrim Health Care Institute’s Department ofPopulation Medicine, an affiliate of Harvard Medical School, and BostonUniversity published their findings in the February 2010 issue of Obstetrics& Gynecology.

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

But that’s not what’s been happening in the past 15 years, at least, saysco-author Emily Oken, MD, MPH, an assistant professor of medicine at HarvardMedical School.

She says her research team expected to find that birth weights were stillincreasing, partly because of increasing age of mothers giving birth anddecreased rates of smoking.

Instead, the researchers say they found that birth weights of full-termbabies decreased by an average of 1.83 ounces between 1990 and 2005, and thatdecreases were especially notable after 1995.

The researchers analyzed data on birth weight, maternal and neonatalcharacteristics, obstetric care, and other trends from the National Center forHealth Statistics, looking at records of 36.8 million single-birth babies bornfull 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 andethnicity, education level, marital status, tobacco use, and the amount ofweight the women had gained during pregnancy.

The researchers also took into account the risk of hypertension and use ofprocedures such as induction of labor and cesarean delivery.

Birth weights were even lower for babies born to women considered to be atlow risk of having small babies, the researchers say.

Mothers who were white, well educated, married, didn’t smoke, and receivedearly prenatal care had babies weighing an average of 2.78 ounces less at birthin 2005 compared to 1990, the authors note.

Average pregnancy time among full-term births also dropped by more than twodays, the researchers say. The researchers note that factors such as thedecreased pregnancy time and increased use of C-sections for delivery do notaccount for the declines in newborn weight.

The decline in birth weights may represent a reversal of previous increasesand needs further investigation, the researchers say.

The researchers say additional studies may identify other factors that mightcontribute 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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