US Newborns Get Smaller Mysterious drop most acute with mothers considered least at risk By Harry Kimball, Newser Staff Posted Jan 22, 2010 2:38 PM CST 29 comments Comments A newborn on a scale. (Shutterstock) (Newser) – The birth weight of the average full-term baby born in the US decreased by 1.83 ounces between 1990 and 2005. Researchers discovered the trend, which runs counter to received notions about increasing birth weight, after studying the records of 37 million newborns. Even more surprising, babies delivered by the group at the lowest risk for low-birthweight infants—white, well-educated nonsmokers with good prenatal care—had the biggest drop in weight, 2.78 ounces, reports LiveScience. "We were startled by the findings," the senior author of the National Institutes of Health study tells the Los Angeles Times. "We tried really hard to explain it away, but we were unable to."