U.S. birth weights dropping

BOSTON, Jan. 22 (UPI) -- U.S. newborns have gotten smaller during the past 15 years, reversing a decades-long upward trend in birth weights, Harvard researchers say.

Researchers at the Harvard Pilgrim Health Care Institute's department of population medicine, an affiliate of Harvard Medical School, analyzed data on birth weight, maternal and neonatal characteristics, obstetric care and other trends from the National Center for Health Statistics Natality Data Sets. The researchers looked at data from 36,827,828 U.S. babies born at full-term between 1990 and 2005.

"Previous studies [covering periods into the early 1990s] have shown that birth weights have increased steadily during the past half-century," Emily Oken of Harvard Medical School said in a statement. "We expected to see a continuation of those increases."

Instead, Oken and her colleagues found birth weights had decreased by an average of 1.83 ounces between 1990 and 2005.

The study, published in February's Obstetrics & Gynecology, also found white, well-educated, married women who didn't smoke, received early prenatal care and delivered vaginally with no complications had babies who weighed an average of 2.78 ounces less at birth during the study period.

The causes of this decline remain unclear and babies born small not only face short-term complications but increased risk as adults.

Future research may identify other factors not included in the current data that might contribute to lower birth weight, such as trends in mothers' diets, physical activity, stress and exposure to environmental toxins.